@@ -1050,16 +1050,23 @@ IonBuilder::inlineMathAbs(CallInfo& call
 {
     if (callInfo.argc() != 1 || callInfo.constructing()) {
         trackOptimizationOutcome(TrackedOutcome::CantInlineNativeBadForm);
         return InliningStatus_NotInlined;
     }

     MIRType returnType = getInlineReturnType();
     MIRType argType = callInfo.getArg(0)->type();
+
+    if (argType == MIRType::Undefined) {
+        callInfo.setImplicitlyUsedUnchecked();
+        pushConstant(DoubleValue(GenericNaN()));
+        return InliningStatus_Inlined;
+    }
+
     if (!IsNumberType(argType))
         return InliningStatus_NotInlined;

     // Either argType == returnType, or
     //        argType == Double or Float32, returnType == Int, or
     //        argType == Float32, returnType == Double
     if (argType != returnType && !(IsFloatingPointType(argType) && returnType == MIRType::Int32)
         && !(argType == MIRType::Float32 && returnType == MIRType::Double))