diff --git a/src/cc65/function.c b/src/cc65/function.c
index d4409afba..bed87a574 100644
--- a/src/cc65/function.c
+++ b/src/cc65/function.c
@@ -49,6 +49,7 @@
 #include "scanner.h"
 #include "segments.h"
 #include "stackptr.h"
+#include "standard.h"
 #include "stmt.h"
 #include "symtab.h"
 #include "function.h"
@@ -373,6 +374,13 @@ void NewFunc (SymEntry* Func)
             Error ("`main' cannot be declared as __fastcall__");
         }
 
+        /* If cc65 extensions aren't enabled, don't allow a main function that 
+         * doesn't return an int.
+         */
+        if (IS_Get (&Standard) != STD_CC65 && CurrentFunc->ReturnType[0] != T_INT) {
+            Error ("`main' must always return an int");
+        }
+
         /* If main() takes parameters, generate a forced import to a function
          * that will setup these parameters. This way, programs that do not
          * need the additional code will not get it.