diff --git a/src/vim9.h b/src/vim9.h
index b0c465d..c51be8e 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -224,7 +224,8 @@
 // arguments to ISN_CHECKTYPE
 typedef struct {
     type_T	*ct_type;
-    int		ct_off;	    // offset in stack, -1 is bottom
+    char	ct_off;		// offset in stack, -1 is bottom
+    char	ct_arg_idx;	// argument index or zero
 } checktype_T;
 
 // arguments to ISN_STORENR