diff --git a/src/structs.h b/src/structs.h
index dba9be6..0899ae7 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1373,6 +1373,7 @@
 
 #define TTFLAG_VARARGS	1	    // func args ends with "..."
 #define TTFLAG_OPTARG	2	    // func arg type with "?"
+#define TTFLAG_BOOL_OK	4	    // can be converted to bool
 
 /*
  * Structure to hold an internal variable without a name.