diff --git a/src/mbyte.c b/src/mbyte.c
index 8ac15d3..b2519ec 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -5587,7 +5587,8 @@
     void
 f_charclass(typval_T *argvars, typval_T *rettv UNUSED)
 {
-    if (check_for_string_arg(argvars, 0) == FAIL)
+    if (check_for_string_arg(argvars, 0) == FAIL
+	    || argvars[0].vval.v_string == NULL)
 	return;
     rettv->vval.v_number = mb_get_class(argvars[0].vval.v_string);
 }