diff --git a/src/utils/utf8.php b/src/utils/utf8.php
--- a/src/utils/utf8.php
+++ b/src/utils/utf8.php
@@ -789,18 +789,22 @@
  * @phutil-external-symbol function mb_convert_case
  */
 function phutil_utf8_strtolower($str) {
-  if (function_exists('mb_convert_case')) {
-    return mb_convert_case($str, MB_CASE_LOWER, 'UTF-8');
-  }
+  if (phutil_nonempty_stringlike($str)) {
+    if (function_exists('mb_convert_case')) {
+      return mb_convert_case($str, MB_CASE_LOWER, 'UTF-8');
+    }
 
-  static $map;
-  if ($map === null) {
-    $map = array_combine(
-      range('A', 'Z'),
-      range('a', 'z'));
-  }
+    static $map;
+    if ($map === null) {
+      $map = array_combine(
+        range('A', 'Z'),
+        range('a', 'z'));
+    }
 
-  return phutil_utf8_strtr($str, $map);
+    return phutil_utf8_strtr($str, $map);
+  } else {
+    return '';
+  }
 }