diff --git a/src/command.c b/src/command.c
index b9c77b10..a02c77e9 100644
--- a/src/command.c
+++ b/src/command.c
@@ -1660,7 +1660,7 @@ _cmd_set_boolean_preference(const char * const inp, struct cmd_help_t help,
         cons_show(disabled->str);
         set_func(FALSE);
     } else {
-        char usage[strlen(help.usage + 8)];
+        char usage[strlen(help.usage) + 8];
         sprintf(usage, "Usage: %s", help.usage);
         cons_show(usage);
     }