diff --git a/src/moduleutils/PhutilLibraryMapBuilder.php b/src/moduleutils/PhutilLibraryMapBuilder.php
--- a/src/moduleutils/PhutilLibraryMapBuilder.php
+++ b/src/moduleutils/PhutilLibraryMapBuilder.php
@@ -196,6 +196,11 @@
    */
   private function writeSymbolCache(array $symbol_map, array $source_map) {
     $cache_file = $this->getPathForSymbolCache();
+    try {
+      Filesystem::assertWritableFile($cache_file);
+    } catch (Exception $ex) {
+      return;
+    }
 
     $cache = array(
       self::SYMBOL_CACHE_VERSION_KEY => self::SYMBOL_CACHE_VERSION,