diff --git a/useful_functions.c b/useful_functions.c
index e0b1e4c..a36062e 100644
--- a/useful_functions.c
+++ b/useful_functions.c
@@ -151,6 +151,8 @@
  */
 int ebt_check_inverse(const char option[])
 {
+	if (!option)
+		return ebt_invert;
 	if (strcmp(option, "!") == 0) {
 		if (ebt_invert == 1)
 			ebt_print_error("double use of '!' not allowed");