diff --git a/src/util-rule-vars.c b/src/util-rule-vars.c
index f3b5604fbe..57347f4ff8 100644
--- a/src/util-rule-vars.c
+++ b/src/util-rule-vars.c
@@ -75,7 +75,7 @@ char *SCRuleVarsGetConfVar(const DetectEngineCtx *de_ctx,
     if (conf_var_name == NULL)
         goto end;
 
-    while (conf_var_name[0] != '\0' && isspace(conf_var_name[0])) {
+    while (conf_var_name[0] != '\0' && isspace((unsigned char)conf_var_name[0])) {
         conf_var_name++;
     }