diff --git a/src/XenoAtom.Allocators.Tests/BasicTests.cs b/src/XenoAtom.Allocators.Tests/BasicTests.cs
index 3a82863..08be0c4 100644
--- a/src/XenoAtom.Allocators.Tests/BasicTests.cs
+++ b/src/XenoAtom.Allocators.Tests/BasicTests.cs
@@ -97,6 +97,8 @@ public async Task TestAllocate3()
 
         // Free allocation 1
         tlsf.Free(allocation1);
+        Assert.ThrowsException<ArgumentException>(() => tlsf.Free(allocation1));
+
         Assert.AreEqual(2U, tlsf.Chunks[0].UsedBlockCount);
         Assert.AreEqual(2U, tlsf.Chunks[0].FreeBlockCount);
 
diff --git a/src/XenoAtom.Allocators/TlsfAllocator.cs b/src/XenoAtom.Allocators/TlsfAllocator.cs
index 5f6d0e8..e46142c 100644
--- a/src/XenoAtom.Allocators/TlsfAllocator.cs
+++ b/src/XenoAtom.Allocators/TlsfAllocator.cs
@@ -179,8 +179,15 @@ public TlsfAllocation Allocate(uint size)
     public void Free(TlsfAllocation allocation)
     {
         int blockIndex = (int)allocation.BlockIndex;
+        if ((uint)blockIndex >= (uint)_blockCount)
+        {
+            throw new ArgumentException($"The block index {blockIndex} is out of range", nameof(allocation));
+        }
         ref var block = ref GetBlockAt(blockIndex);
-        Debug.Assert(block.IsUsed);
+        if (!block.IsUsed)
+        {
+            throw new ArgumentException($"The block at index {blockIndex} is already free", nameof(allocation));
+        }
         block.IsUsed = false;
 
         // Update statistics for the chunk