diff --git a/src/basic/util.c b/src/basic/util.c
index a9071aa1178..737f2a221c6 100644
--- a/src/basic/util.c
+++ b/src/basic/util.c
@@ -4273,7 +4273,7 @@ bool is_locale_utf8(void) {
         /* Check result, but ignore the result if C was set
          * explicitly. */
         cached_answer =
-                streq(set, "C") &&
+                STR_IN_SET(set, "C", "POSIX") &&
                 !getenv("LC_ALL") &&
                 !getenv("LC_CTYPE") &&
                 !getenv("LANG");