diff --git a/include/types/acl.h b/include/types/acl.h
index d67928897..0eecd335f 100644
--- a/include/types/acl.h
+++ b/include/types/acl.h
@@ -164,6 +164,7 @@ struct acl_expr {
 	union {                     /* optional argument of the subject (eg: header or cookie name) */
 		char *str;
 	} arg;
+	int arg_len;                /* optional argument length */
 	struct list patterns;       /* list of acl_patterns */
 };
 
diff --git a/src/acl.c b/src/acl.c
index 4443ae9ad..2cd154ca0 100644
--- a/src/acl.c
+++ b/src/acl.c
@@ -435,6 +435,7 @@ struct acl_expr *parse_acl_expr(const char **args)
 	aclkw->use_cnt++;
 	LIST_INIT(&expr->patterns);
 	expr->arg.str = NULL;
+	expr->arg_len = 0;
 
 	arg = strchr(args[0], '(');
 	if (arg != NULL) {
@@ -449,6 +450,7 @@ struct acl_expr *parse_acl_expr(const char **args)
 			goto out_free_expr;
 		memcpy(arg2, arg, end - arg);
 		arg2[end-arg] = '\0';
+		expr->arg_len = end - arg;
 		expr->arg.str = arg2;
 	}