diff --git a/toolkit/mozapps/downloads/content/nsHelperAppDlg.js b/toolkit/mozapps/downloads/content/nsHelperAppDlg.js
index 017a6844235..94b153e117d 100755
--- a/toolkit/mozapps/downloads/content/nsHelperAppDlg.js
+++ b/toolkit/mozapps/downloads/content/nsHelperAppDlg.js
@@ -377,9 +377,16 @@ nsUnknownContentTypeDialog.prototype = {
         var typeString = mimeInfo.Description;
         
         if (typeString == "") {
-          // 2. If there is none, use the extension to identify the file, e.g. "ZIP file"
-          if (mimeInfo.primaryExtension != "")
-            typeString = mimeInfo.primaryExtension.toUpperCase() + " file";
+          // 2. If there is none, use the extension to identify the file, e.g. "ZIP file"
+          var primaryExtension = "";
+          try {
+            primaryExtension = mimeInfo.primaryExtension;
+          }
+          catch (ex) {
+          }
+
+          if (primaryExtension != "")
+            typeString = primaryExtension.toUpperCase() + " file";
           // 3. If we can't even do that, just give up and show the MIME type. 
           else
             typeString = mimeInfo.MIMEType;