diff --git a/src/shared/ask-password-api.c b/src/shared/ask-password-api.c
index 9aad834ee28..729aa1fb00a 100644
--- a/src/shared/ask-password-api.c
+++ b/src/shared/ask-password-api.c
@@ -581,10 +581,10 @@ int ask_password_tty(
                                  * last one begins */
                                 q = 0;
                                 for (;;) {
-                                        size_t z;
+                                        int z;
 
                                         z = utf8_encoded_valid_unichar(passphrase + q, SIZE_MAX);
-                                        if (z == 0) {
+                                        if (z <= 0) {
                                                 q = SIZE_MAX; /* Invalid UTF8! */
                                                 break;
                                         }