--- a/toolkit/mozapps/extensions/internal/XPIProvider.jsm
+++ b/toolkit/mozapps/extensions/internal/XPIProvider.jsm
@@ -958,25 +958,27 @@ function getRDFProperty(aDs, aResource, 
 var loadManifestFromWebManifest = Task.async(function*(aUri) {
   // We're passed the URI for the manifest file. Get the URI for its
   // parent directory.
   let uri = NetUtil.newURI("./", null, aUri);
 
   let extension = new ExtensionData(uri);
 
   let manifest = yield extension.loadManifest();
-  let theme = !!manifest.theme;
 
   // Read the list of available locales, and pre-load messages for
   // all locales.
-  let locales = yield extension.initAllLocales();
-
-  // If there were any errors loading the extension, bail out now.
-  if (extension.errors.length)
+  let locales = (extension.errors.length == 0) ?
+                yield extension.initAllLocales() : null;
+
+  if (extension.errors.length > 0) {
     throw new Error("Extension is invalid");
+  }
+
+  let theme = Boolean(manifest.theme);
 
   let bss = (manifest.browser_specific_settings && manifest.browser_specific_settings.gecko)
       || (manifest.applications && manifest.applications.gecko) || {};
   if (manifest.browser_specific_settings && manifest.applications) {
     logger.warn("Ignoring applications property in manifest");
   }
 
   // A * is illegal in strict_min_version