diff --git a/file_aliases.module b/file_aliases.module
index 301e268..70e628b 100755
--- a/file_aliases.module
+++ b/file_aliases.module
@@ -45,9 +45,11 @@ function file_aliases_file_load($files) {
 function file_aliases_preprocess_image_style(&$variables) {
   global $base_url;
   $source = drupal_lookup_path('source', str_replace($base_url . base_path(), '', $variables['path']));
-  $fid = drupal_substr($source, 22);
-  if ($uri = db_query("SELECT uri FROM {file_managed} WHERE fid = :fid", array(':fid' => $fid))->fetchField()) {
-    $variables['path'] = $uri;
+  if ($source !== FALSE) {
+    $fid = drupal_substr($source, 22);
+    if ($uri = db_query("SELECT uri FROM {file_managed} WHERE fid = :fid", array(':fid' => $fid))->fetchField()) {
+      $variables['path'] = $uri;
+    }
   }
 }