diff --git a/terminal.c b/terminal.c
index dee0b038..ecf21040 100644
--- a/terminal.c
+++ b/terminal.c
@@ -3528,7 +3528,7 @@ term_set_window_title(struct terminal *term, const char *title)
     if (term->window_title != NULL && streq(term->window_title, title))
         return;
 
-    if (mbsntoc32(NULL, title, strlen(title), 0) == (char32_t)-1) {
+    if (mbsntoc32(NULL, title, strlen(title), 0) == (size_t)-1) {
         /* It's an xdg_toplevel::set_title() protocol violation to set
            a title with an invalid UTF-8 sequence */
         LOG_WARN("%s: title is not valid UTF-8, ignoring", title);