diff --git a/docs/design/datacontracts/GCInfo.md b/docs/design/datacontracts/GCInfo.md
index 7ba3cfbc90ab9d..a136cffdac2dbc 100644
--- a/docs/design/datacontracts/GCInfo.md
+++ b/docs/design/datacontracts/GCInfo.md
@@ -665,6 +665,9 @@ For encodings without interruptible ranges (`HAS_INTERRUPTIBLE_RANGES` is false,
 
 **Slot filtering**: Before reporting any slot, the algorithm checks:
 - **Scratch registers**: Only reported for the active/leaf frame (`ActiveStackFrame` flag).
+  Valid Wasm GC info never contains register slots, so the WebAssembly platform
+  decoder's scratch-register predicate always returns false and asserts in debug
+  builds if called, mirroring the native unsupported-platform fallback.
 - **Scratch stack slots**: Only reported for the active/leaf frame (slots in the outgoing/scratch area).
 - **FP-based-only mode** (`ReportFPBasedSlotsOnly`): Only frame-register-relative stack slots are reported; all register slots and non-frame-relative stack slots are skipped.
 
diff --git a/src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/GCInfo/PlatformTraits/WasmGCInfoTraits.cs b/src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/GCInfo/PlatformTraits/WasmGCInfoTraits.cs
index 60fa8fe8f3894b..6097373872693b 100644
--- a/src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/GCInfo/PlatformTraits/WasmGCInfoTraits.cs
+++ b/src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/GCInfo/PlatformTraits/WasmGCInfoTraits.cs
@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+
 namespace Microsoft.Diagnostics.DataContractReader.Contracts.GCInfoHelpers;
 
 // Mirrors Wasm32GcInfoEncoding in src/coreclr/inc/gcinfotypes.h.
@@ -37,5 +39,13 @@ internal sealed class WasmGCInfoTraits : IGCInfoTraits
     public static int NUM_INTERRUPTIBLE_RANGES_ENCBASE => 1;
     public static bool HAS_FIXED_STACK_PARAMETER_SCRATCH_AREA => false;
     public static bool HAS_INTERRUPTIBLE_RANGES => false;
-    public static bool IsScratchRegister(uint regNum) => false;
+
+    // Valid Wasm GC info has no register slots; RyuJIT never emits one. Mirror the
+    // native fallback (gcinfodecoder.cpp PORTABILITY_ASSERT): flag the unexpected
+    // call in debug builds and otherwise return false, like other unimplemented platforms.
+    public static bool IsScratchRegister(uint regNum)
+    {
+        Debug.Fail("Wasm GC info must not contain register slots.");
+        return false;
+    }
 }