diff --git a/src/cc65/datatype.c b/src/cc65/datatype.c
index 6907ee099..103a3a634 100644
--- a/src/cc65/datatype.c
+++ b/src/cc65/datatype.c
@@ -670,6 +670,10 @@ const Type* ArithmeticConvert (const Type* lhst, const Type* rhst)
     ** floating point types are not (yet) supported.
     ** The integral promotions are performed on both operands.
     */
+    if (IsClassFloat(lhst) || IsClassFloat(rhst)) {
+	    Error ("Floating point arithmetic not supported.");
+		return type_long;
+    }
     lhst = IntPromotion (lhst);
     rhst = IntPromotion (rhst);