diff --git a/src/return.c b/src/return.c
index 0872753..0b8a23b 100644
--- a/src/return.c
+++ b/src/return.c
@@ -94,7 +94,7 @@ void contextsw(DLword fxnum, DLword bytenum, DLword flags)
   printf("contextsw : %d \n", fxnum);
 #endif
 
-  if (!(fxnum == SubovFXP)) {
+  if (fxnum != SubovFXP) {
     /* interrupt disable during executing [special] function
       invoked by contextsw(\KEYHANDLER,\RESETSTACK,FAULT)
      */