diff --git a/src/basic/utf8.c b/src/basic/utf8.c
index 7b47df6f4f..633d86c6cf 100644
--- a/src/basic/utf8.c
+++ b/src/basic/utf8.c
@@ -613,6 +613,8 @@ size_t utf8_console_width(const char *str) {
 size_t utf8_last_length(const char *s, size_t n) {
         int r;
 
+        assert(s);
+
         if (n == SIZE_MAX)
                 n = strlen(s);