diff --git a/src/thingdef/thingdef_expression.cpp b/src/thingdef/thingdef_expression.cpp
index b5b35a770..d4bb41326 100644
--- a/src/thingdef/thingdef_expression.cpp
+++ b/src/thingdef/thingdef_expression.cpp
@@ -3518,7 +3518,7 @@ FxFlopFunctionCall::FxFlopFunctionCall(size_t index, FArgumentList *args, const
 : FxExpression(pos)
 {
 	assert(index < countof(FxFlops) && "FLOP index out of range");
-	Index = index;
+	Index = (int)index;
 	ArgList = args;
 }