diff --git a/lib/pam_set_item.c b/lib/pam_set_item.c
index 1cebfd5..cc6f9c4 100644
--- a/lib/pam_set_item.c
+++ b/lib/pam_set_item.c
@@ -72,7 +72,8 @@ pam_set_item(pam_handle_t *pamh,
 	case PAM_RUSER:
 	case PAM_USER_PROMPT:
 	case PAM_AUTHTOK_PROMPT:
-		size = strlen(*slot) + 1;
+		if (*slot != NULL)
+			size = strlen(*slot) + 1;
 		if (item != NULL)
 			tmp = strdup(item);
 		break;