diff --git a/src/index.ts b/src/index.ts
index fd6d706..f2ca8a3 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -89,7 +89,9 @@ export function createRunner<TestComponents extends Record<string, any>>(
     function spy<T extends {}>(module: T): SpiedInstance<T> {
       const ret = {} as SpiedInstance<T>
       for (let key in module) {
-        ret[key as keyof T] = jest.spyOn(module, key as any) as any
+        if (typeof module[key] === "function") {
+          ret[key as keyof T] = jest.spyOn(module, key as any) as any
+        }
       }
       return ret
     }
diff --git a/test/simple-smoke-test/jest.spec.ts b/test/simple-smoke-test/jest.spec.ts
index e6f569c..78ac106 100644
--- a/test/simple-smoke-test/jest.spec.ts
+++ b/test/simple-smoke-test/jest.spec.ts
@@ -8,6 +8,7 @@ type Components = {
   componentB: {
     sum(a: number, b: number): number
     counter(): number
+    t: number
   }
 }
 
@@ -31,6 +32,7 @@ const test = createRunner<Components>({
       counter() {
         return counter
       },
+      t: 3
     }
 
     return { componentA, componentB }