diff --git a/src/moduleutils/PhutilLibraryMapBuilder.php b/src/moduleutils/PhutilLibraryMapBuilder.php
--- a/src/moduleutils/PhutilLibraryMapBuilder.php
+++ b/src/moduleutils/PhutilLibraryMapBuilder.php
@@ -122,7 +122,7 @@
    */
   private function log($message) {
     if (!$this->quiet) {
-      @fwrite(STDERR, "%s\n", $message);
+      @fprintf(STDERR, "%s\n", $message);
     }
     return $this;
   }