diff --git a/include/swift/Reflection/TypeRefBuilder.h b/include/swift/Reflection/TypeRefBuilder.h
index daf75a68b3e89..b0458c6896cf0 100644
--- a/include/swift/Reflection/TypeRefBuilder.h
+++ b/include/swift/Reflection/TypeRefBuilder.h
@@ -571,8 +571,13 @@ class TypeRefBuilder {
 
     auto numGenericArgs = genericParamsPerLevel[shapeIndex];
 
+    auto startOffsetFromEnd = argsIndex + numGenericArgs;
+    auto endOffsetFromEnd = argsIndex;
+    if (startOffsetFromEnd > args.size() || endOffsetFromEnd > args.size())
+      return nullptr;
+
     std::vector<const TypeRef *> genericParams(
-        args.end() - argsIndex - numGenericArgs, args.end() - argsIndex);
+        args.end() - startOffsetFromEnd, args.end() - endOffsetFromEnd);
 
     const BoundGenericTypeRef *parent = nullptr;
     if (node->hasChildren()) {