diff --git a/src/error/PhutilOpaqueEnvelope.php b/src/error/PhutilOpaqueEnvelope.php
index 1c9024ba..ef773fb1 100644
--- a/src/error/PhutilOpaqueEnvelope.php
+++ b/src/error/PhutilOpaqueEnvelope.php
@@ -60,11 +60,13 @@ final class PhutilOpaqueEnvelope extends Phobject {
    */
   private function mask($string, $noise) {
     $result = '';
-    for ($ii = 0; $ii < strlen($string); $ii++) {
-      $s = $string[$ii];
-      $n = $noise[$ii % strlen($noise)];
+    if (phutil_nonempty_string($string)) {
+      for ($ii = 0; $ii < strlen($string); $ii++) {
+        $s = $string[$ii];
+        $n = $noise[$ii % strlen($noise)];
 
-      $result .= chr(ord($s) ^ ord($n));
+        $result .= chr(ord($s) ^ ord($n));
+      }
     }
     return $result;
   }