diff --git a/src/pam_weblogin.c b/src/pam_weblogin.c
index 64fccec..6dc6dbc 100644
--- a/src/pam_weblogin.c
+++ b/src/pam_weblogin.c
@@ -233,7 +233,13 @@ PAM_EXTERN int pam_sm_authenticate(pam_handle_t *pamh, UNUSED int flags, int arg
 							tty_output(pamh, str_printf("  [%d] %s", i+1, name));
 						}
 						char *group_input = tty_input(pamh, PROMPT_GROUP, PAM_PROMPT_ECHO_ON);
-						group = strtol(group_input, &end, 10);
+						if (!group_input)
+						{
+							errno = ERANGE; /* treat no input as invalid input */
+						} else
+						{
+							group = strtol(group_input, &end, 10);
+						}
 						free(group_input);
 						range_error = errno == ERANGE;
 						if (group < 1 || group > max_groups || range_error)