diff --git a/src/protocols/vnc/vnc.c b/src/protocols/vnc/vnc.c
index 66de2634..a3ae9b12 100644
--- a/src/protocols/vnc/vnc.c
+++ b/src/protocols/vnc/vnc.c
@@ -107,7 +107,7 @@ static rfbBool guac_vnc_unlock_write_to_tls(rfbClient* rfb_client) {
     /* Unlock write access */
     int retval = pthread_mutex_unlock(&(vnc_client->tls_lock));
     if (retval) {
-        guac_client_log(gc, GUAC_LOG_ERROR, "Error unlocking TLS write mutex: %d",
+        guac_client_log(gc, GUAC_LOG_ERROR, "Error unlocking TLS write mutex: %s",
                 strerror(retval));
         return FALSE;
     }