diff --git a/packages/lib/src/gir.ts b/packages/lib/src/gir.ts
index c0153a52..06b3b5aa 100644
--- a/packages/lib/src/gir.ts
+++ b/packages/lib/src/gir.ts
@@ -16,7 +16,7 @@ export { promisifyFunctions } from './gir/promisify.js'
 export { resolveDirectedType, resolvePrimitiveType } from './gir/util.js'
 export * from './gir/nodes.js'
 
-import type { OptionsGeneration } from './types/index.js'
+import type { OptionsBase } from './types/index.js'
 
 export abstract class TypeExpression {
     isPointer = false
@@ -29,10 +29,10 @@ export abstract class TypeExpression {
     }
 
     abstract rewrap(type: TypeExpression): TypeExpression
-    abstract resolve(namespace: IntrospectedNamespace, options: OptionsGeneration): TypeExpression
+    abstract resolve(namespace: IntrospectedNamespace, options: OptionsBase): TypeExpression
 
-    abstract print(namespace: IntrospectedNamespace, options: OptionsGeneration): string
-    rootPrint(namespace: IntrospectedNamespace, options: OptionsGeneration): string {
+    abstract print(namespace: IntrospectedNamespace, options: OptionsBase): string
+    rootPrint(namespace: IntrospectedNamespace, options: OptionsBase): string {
         return this.print(namespace, options)
     }
 }
@@ -72,7 +72,7 @@ export class TypeIdentifier extends TypeExpression {
         return new TypeIdentifier(sanitizeIdentifierName(this.namespace, this.name), sanitizeNamespace(this.namespace))
     }
 
-    protected _resolve(namespace: IntrospectedNamespace, options: OptionsGeneration): TypeIdentifier | null {
+    protected _resolve(namespace: IntrospectedNamespace, options: OptionsBase): TypeIdentifier | null {
         const name: string = sanitizeIdentifierName(null, this.name)
         const unresolvedNamespaceName = this.namespace
 
@@ -144,11 +144,11 @@ export class TypeIdentifier extends TypeExpression {
         return null
     }
 
-    resolveIdentifier(namespace: IntrospectedNamespace, options: OptionsGeneration): TypeIdentifier | null {
+    resolveIdentifier(namespace: IntrospectedNamespace, options: OptionsBase): TypeIdentifier | null {
         return this._resolve(namespace, options)
     }
 
-    resolve(namespace: IntrospectedNamespace, options: OptionsGeneration): TypeExpression {
+    resolve(namespace: IntrospectedNamespace, options: OptionsBase): TypeExpression {
         const resolved = this._resolve(namespace, options)
 
         // Generally if we can't resolve a type it is not introspectable,
@@ -161,7 +161,7 @@ export class TypeIdentifier extends TypeExpression {
     }
 
     // eslint-disable-next-line  @typescript-eslint/no-unused-vars
-    print(namespace: IntrospectedNamespace, _options: OptionsGeneration): string {
+    print(namespace: IntrospectedNamespace, _options: OptionsBase): string {
         if (namespace.hasSymbol(this.namespace) && this.namespace !== namespace.namespace) {
             // TODO: Move to TypeScript generator...
             // Libraries like zbar have classes named things like "Gtk"
@@ -212,12 +212,12 @@ export class ModuleTypeIdentifier extends TypeIdentifier {
     }
 
     // eslint-disable-next-line  @typescript-eslint/no-unused-vars
-    protected _resolve(namespace: IntrospectedNamespace, options: OptionsGeneration): ModuleTypeIdentifier | null {
+    protected _resolve(namespace: IntrospectedNamespace, options: OptionsBase): ModuleTypeIdentifier | null {
         return this
     }
 
     // eslint-disable-next-line  @typescript-eslint/no-unused-vars
-    print(namespace: IntrospectedNamespace, options: OptionsGeneration): string {
+    print(namespace: IntrospectedNamespace, options: OptionsBase): string {
         if (namespace.namespace === this.namespace) {
             return `${this.moduleName}.${this.name}`
         } else {
@@ -239,7 +239,7 @@ export class ClassStructTypeIdentifier extends TypeIdentifier {
     }
 
     // eslint-disable-next-line  @typescript-eslint/no-unused-vars
-    print(namespace: IntrospectedNamespace, options: OptionsGeneration): string {
+    print(namespace: IntrospectedNamespace, options: OptionsBase): string {
         if (namespace.namespace === this.namespace) {
             // TODO: Mapping to invalid names should happen at the generator level...
             return `typeof ${isInvalid(this.name) ? `__${this.name}` : this.name}`
@@ -257,7 +257,7 @@ export class GenerifiedTypeIdentifier extends TypeIdentifier {
         this.generics = generics
     }
 
-    print(namespace: IntrospectedNamespace, options: OptionsGeneration): string {
+    print(namespace: IntrospectedNamespace, options: OptionsBase): string {
         const Generics = this.generics.map((generic) => generic.print(namespace, options)).join(', ')
 
         if (namespace.namespace === this.namespace) {
@@ -267,7 +267,7 @@ export class GenerifiedTypeIdentifier extends TypeIdentifier {
         }
     }
 
-    _resolve(namespace: IntrospectedNamespace, options: OptionsGeneration): TypeIdentifier | null {
+    _resolve(namespace: IntrospectedNamespace, options: OptionsBase): TypeIdentifier | null {
         const iden = super._resolve(namespace, options)
 
         if (iden) {
@@ -279,9 +279,9 @@ export class GenerifiedTypeIdentifier extends TypeIdentifier {
 }
 
 export class NativeType extends TypeExpression {
-    readonly expression: (options?: OptionsGeneration) => string
+    readonly expression: (options?: OptionsBase) => string
 
-    constructor(expression: ((options?: OptionsGeneration) => string) | string) {
+    constructor(expression: ((options?: OptionsBase) => string) | string) {
         super()
 
         this.expression = typeof expression === 'string' ? () => expression : expression
@@ -295,11 +295,11 @@ export class NativeType extends TypeExpression {
         return this
     }
 
-    print(_namespace: IntrospectedNamespace, options: OptionsGeneration) {
+    print(_namespace: IntrospectedNamespace, options: OptionsBase) {
         return this.expression(options)
     }
 
-    equals(type: TypeExpression, options?: OptionsGeneration): boolean {
+    equals(type: TypeExpression, options?: OptionsBase): boolean {
         return type instanceof NativeType && this.expression(options) === type.expression(options)
     }
 
@@ -307,7 +307,7 @@ export class NativeType extends TypeExpression {
         return this
     }
 
-    static withGenerator(generator: (options?: OptionsGeneration) => string): TypeExpression {
+    static withGenerator(generator: (options?: OptionsBase) => string): TypeExpression {
         return new NativeType(generator)
     }
 
@@ -332,17 +332,17 @@ export class OrType extends TypeExpression {
         return this
     }
 
-    resolve(namespace: IntrospectedNamespace, options: OptionsGeneration): TypeExpression {
+    resolve(namespace: IntrospectedNamespace, options: OptionsBase): TypeExpression {
         const [type, ...types] = this.types
 
         return new OrType(type.resolve(namespace, options), ...types.map((t) => t.resolve(namespace, options)))
     }
 
-    print(namespace: IntrospectedNamespace, options: OptionsGeneration): string {
+    print(namespace: IntrospectedNamespace, options: OptionsBase): string {
         return `(${this.types.map((t) => t.print(namespace, options)).join(' | ')})`
     }
 
-    rootPrint(namespace: IntrospectedNamespace, options: OptionsGeneration): string {
+    rootPrint(namespace: IntrospectedNamespace, options: OptionsBase): string {
         return `${this.types.map((t) => t.print(namespace, options)).join(' | ')}`
     }
 
@@ -356,15 +356,15 @@ export class OrType extends TypeExpression {
 }
 
 export class TupleType extends OrType {
-    print(namespace: IntrospectedNamespace, options: OptionsGeneration): string {
+    print(namespace: IntrospectedNamespace, options: OptionsBase): string {
         return `[${this.types.map((t) => t.print(namespace, options)).join(', ')}]`
     }
 
-    rootPrint(namespace: IntrospectedNamespace, options: OptionsGeneration): string {
+    rootPrint(namespace: IntrospectedNamespace, options: OptionsBase): string {
         return this.print(namespace, options)
     }
 
-    resolve(namespace: IntrospectedNamespace, options: OptionsGeneration): TypeExpression {
+    resolve(namespace: IntrospectedNamespace, options: OptionsBase): TypeExpression {
         const [type, ...types] = this.types
 
         return new TupleType(type.resolve(namespace, options), ...types.map((t) => t.resolve(namespace, options)))
@@ -388,7 +388,7 @@ export class BinaryType extends OrType {
         return this
     }
 
-    resolve(namespace: IntrospectedNamespace, options: OptionsGeneration) {
+    resolve(namespace: IntrospectedNamespace, options: OptionsBase) {
         return new BinaryType(this.a.resolve(namespace, options), this.b.resolve(namespace, options))
     }
 
@@ -440,7 +440,7 @@ export class FunctionType extends TypeExpression {
         return this
     }
 
-    resolve(namespace: IntrospectedNamespace, options: OptionsGeneration): TypeExpression {
+    resolve(namespace: IntrospectedNamespace, options: OptionsBase): TypeExpression {
         return new FunctionType(
             Object.fromEntries(
                 Object.entries(this.parameterTypes).map(([k, p]) => {
@@ -451,7 +451,7 @@ export class FunctionType extends TypeExpression {
         )
     }
 
-    rootPrint(namespace: IntrospectedNamespace, options: OptionsGeneration): string {
+    rootPrint(namespace: IntrospectedNamespace, options: OptionsBase): string {
         const Parameters = Object.entries(this.parameterTypes)
             .map(([k, v]) => {
                 return `${k}: ${v.rootPrint(namespace, options)}`
@@ -461,7 +461,7 @@ export class FunctionType extends TypeExpression {
         return `(${Parameters}) => ${this.returnType.print(namespace, options)}`
     }
 
-    print(namespace: IntrospectedNamespace, options: OptionsGeneration): string {
+    print(namespace: IntrospectedNamespace, options: OptionsBase): string {
         return `(${this.rootPrint(namespace, options)})`
     }
 }
@@ -523,7 +523,7 @@ export class GenerifiedType extends TypeExpression {
         this.generic = generic
     }
 
-    resolve(namespace: IntrospectedNamespace, options: OptionsGeneration) {
+    resolve(namespace: IntrospectedNamespace, options: OptionsBase) {
         return new GenerifiedType(this.type.resolve(namespace, options), this.generic.resolve())
     }
 
@@ -531,11 +531,11 @@ export class GenerifiedType extends TypeExpression {
         return this.type
     }
 
-    rootPrint(namespace: IntrospectedNamespace, options: OptionsGeneration) {
+    rootPrint(namespace: IntrospectedNamespace, options: OptionsBase) {
         return `${this.type.print(namespace, options)}<${this.generic.print()}>`
     }
 
-    print(namespace: IntrospectedNamespace, options: OptionsGeneration) {
+    print(namespace: IntrospectedNamespace, options: OptionsBase) {
         return `${this.type.print(namespace, options)}<${this.generic.print()}>`
     }
 
@@ -625,11 +625,11 @@ export class PromiseType extends TypeExpression {
         return new PromiseType(this.type.rewrap(type))
     }
 
-    resolve(namespace: IntrospectedNamespace, options: OptionsGeneration): TypeExpression {
+    resolve(namespace: IntrospectedNamespace, options: OptionsBase): TypeExpression {
         return new PromiseType(this.type.resolve(namespace, options))
     }
 
-    print(namespace: IntrospectedNamespace, options: OptionsGeneration): string {
+    print(namespace: IntrospectedNamespace, options: OptionsBase): string {
         // TODO: Optimize this check.
         if (!namespace.hasSymbol('Promise')) {
             return `Promise<${this.type.print(namespace, options)}>`
@@ -693,7 +693,7 @@ export class TypeConflict extends TypeExpression {
         return true
     }
 
-    resolve(namespace: IntrospectedNamespace, options: OptionsGeneration): TypeExpression {
+    resolve(namespace: IntrospectedNamespace, options: OptionsBase): TypeExpression {
         throw new Error(
             `Type conflict was not resolved for ${this.type.resolve(namespace, options).print(namespace, options)} in ${
                 namespace.namespace
@@ -701,7 +701,7 @@ export class TypeConflict extends TypeExpression {
         )
     }
 
-    print(namespace: IntrospectedNamespace, options: OptionsGeneration): string {
+    print(namespace: IntrospectedNamespace, options: OptionsBase): string {
         throw new Error(
             `Type conflict was not resolved for ${this.type.resolve(namespace, options).print(namespace, options)} in ${
                 namespace.namespace
@@ -744,7 +744,7 @@ export class ClosureType extends TypeExpression {
         return this
     }
 
-    resolve(namespace: IntrospectedNamespace, options: OptionsGeneration) {
+    resolve(namespace: IntrospectedNamespace, options: OptionsBase) {
         const { user_data, type } = this
 
         return ClosureType.new({
@@ -753,7 +753,7 @@ export class ClosureType extends TypeExpression {
         })
     }
 
-    print(namespace: IntrospectedNamespace, options: OptionsGeneration): string {
+    print(namespace: IntrospectedNamespace, options: OptionsBase): string {
         return this.type.print(namespace, options)
     }
 
@@ -801,7 +801,7 @@ export class ArrayType extends TypeExpression {
         return false
     }
 
-    resolve(namespace: IntrospectedNamespace, options: OptionsGeneration): TypeExpression {
+    resolve(namespace: IntrospectedNamespace, options: OptionsBase): TypeExpression {
         const { type, arrayDepth, length } = this
         return ArrayType.new({
             type: type.resolve(namespace, options),
@@ -810,7 +810,7 @@ export class ArrayType extends TypeExpression {
         })
     }
 
-    print(namespace: IntrospectedNamespace, options: OptionsGeneration): string {
+    print(namespace: IntrospectedNamespace, options: OptionsBase): string {
         const depth = this.arrayDepth
         let typeSuffix: string = ''
 
diff --git a/packages/lib/src/gir/registry.ts b/packages/lib/src/gir/registry.ts
index 4c36f4e3..db488517 100644
--- a/packages/lib/src/gir/registry.ts
+++ b/packages/lib/src/gir/registry.ts
@@ -17,6 +17,7 @@ import { ParsedGir } from "../types/parsed-gir.js";
 import { GirModule } from "../index.js";
 
 import type { OptionsGeneration, OptionsTransform } from "../types/index.js";
+import { FunctionParametersVisitor } from "../validators/function-parameters.js";
 
 export interface NSLoader {
     load(namespace: string, version: string): ParsedGir | null;
@@ -147,13 +148,14 @@ export class NSRegistry {
         GObject.package_version = [...GLib.package_version];
 
         const interfaceVisitor = new InterfaceVisitor();
-
         this.registerTransformation(interfaceVisitor);
 
         const classVisitor = new ClassVisitor();
-
         this.registerTransformation(classVisitor);
 
+        const enumParamsVisitor = new FunctionParametersVisitor();
+        this.registerTransformation(enumParamsVisitor);
+
         console.log("Adding generics...");
         generify(this, options.inferGenerics);
 
diff --git a/packages/lib/src/validators/function-parameters.ts b/packages/lib/src/validators/function-parameters.ts
new file mode 100644
index 00000000..fca0542a
--- /dev/null
+++ b/packages/lib/src/validators/function-parameters.ts
@@ -0,0 +1,52 @@
+import { IntrospectedFunction, IntrospectedClassFunction } from "../gir/function.js";
+import { IntrospectedBaseClass } from "../gir/class.js";
+import { IntrospectedEnum } from "../gir/enum.js";
+
+import { TypeIdentifier, NullableType } from "../gir.js";
+import { GirVisitor } from "../visitor.js";
+
+export class FunctionParametersVisitor extends GirVisitor {
+    /**
+     * Marks all enum parameters of a function as nullable,
+     * because GJS allows null values for enum parameters and treats them as a 0 value.
+     * See issue [#207](https://github.com/gjsify/ts-for-gir/issues/207).
+     */
+    private makeEnumParamsNullable(node: IntrospectedFunction): IntrospectedFunction;
+    private makeEnumParamsNullable<T extends IntrospectedBaseClass | IntrospectedEnum>(
+        node: IntrospectedClassFunction<T>
+    ): IntrospectedClassFunction<T>;
+    private makeEnumParamsNullable(
+        node: IntrospectedFunction | IntrospectedClassFunction
+    ): IntrospectedFunction | IntrospectedClassFunction {
+        return node.copy({
+            parameters: node.parameters.map(param => {
+                const type = param.type.deepUnwrap();
+                if (type instanceof TypeIdentifier) {
+                    // Get the namespace where this type should be defined
+                    const ns = node.namespace.assertInstalledImport(type.namespace);
+
+                    // Check if the type is an enum
+                    const isEnumType = !!ns.getEnum(type.name);
+
+                    // If it is, make the parameter nullable
+                    if (isEnumType) {
+                        return param.copy({
+                            type: new NullableType(param.type)
+                        });
+                    }
+                }
+                return param;
+            })
+        });
+    }
+
+    visitFunction = (node: IntrospectedFunction): IntrospectedFunction => {
+        return this.makeEnumParamsNullable(node);
+    };
+
+    visitClassFunction = <T extends IntrospectedBaseClass | IntrospectedEnum>(
+        node: IntrospectedClassFunction<T>
+    ): IntrospectedClassFunction<T> => {
+        return this.makeEnumParamsNullable(node);
+    };
+}