1 files changed, 2 insertions, 1 deletions
diff --git a/source/slang/slang-ir-lower-generic-function.cpp b/source/slang/slang-ir-lower-generic-function.cpp
index bf20452d7..b50737c23 100644
--- a/source/slang/slang-ir-lower-generic-function.cpp
+++ b/source/slang/slang-ir-lower-generic-function.cpp
@@ -264,7 +264,8 @@ namespace Slang
             // If the requirement is a function, interfaceRequirementVal will be the lowered function type.
             // If the requirement is an associatedtype, interfaceRequirementVal will be Ptr<RTTIObject>.
             IRInst* interfaceRequirementVal = nullptr;
-            auto witnessTableType = cast<IRWitnessTableType>(lookupInst->getWitnessTable()->getDataType());
+            auto witnessTableType = as<IRWitnessTableType>(lookupInst->getWitnessTable()->getDataType());
+            if (!witnessTableType) return;
             auto interfaceType = maybeLowerInterfaceType(cast<IRInterfaceType>(witnessTableType->getConformanceType()));
             interfaceRequirementVal = sharedContext->findInterfaceRequirementVal(interfaceType, lookupInst->getRequirementKey());
             lookupInst->setFullType((IRType*)interfaceRequirementVal);