diff --git a/ext/mbstring/php_unicode.c b/ext/mbstring/php_unicode.c
index ac452b6a20776..acb16bf06e4cc 100644
--- a/ext/mbstring/php_unicode.c
+++ b/ext/mbstring/php_unicode.c
@@ -315,7 +315,7 @@ static int convert_case_filter(int c, void *void_data)
 
 	/* Handle invalid characters early, as we assign special meaning to
 	 * codepoints above 0xffffff. */
-	if (UNEXPECTED(c > 0xffffff)) {
+	if (UNEXPECTED((unsigned) c > 0xffffff)) {
 		(*data->next_filter->filter_function)(c, data->next_filter);
 		return 0;
 	}
diff --git a/ext/mbstring/tests/bug79371.phpt b/ext/mbstring/tests/bug79371.phpt
new file mode 100644
index 0000000000000..3014feba5369f
--- /dev/null
+++ b/ext/mbstring/tests/bug79371.phpt
@@ -0,0 +1,14 @@
+--TEST--
+Bug #79371 (mb_strtolower (UTF-32LE): stack-buffer-overflow)
+--SKIPIF--
+<?php
+if (!extension_loaded('mbstring')) die('skip mbstring extension not available');
+?>
+--FILE--
+<?php
+$bytes = array(0xef, 0xbf, 0xbd, 0xef);
+$str = implode(array_map("chr", $bytes));
+var_dump(bin2hex(mb_strtolower($str, "UTF-32LE")));
+?>
+--EXPECT--
+string(8) "3f000000"