diff --git a/toolkit/mozapps/update/src/updater/updater.cpp b/toolkit/mozapps/update/src/updater/updater.cpp
index 0498844f62ae..dd8095cec62d 100644
--- a/toolkit/mozapps/update/src/updater/updater.cpp
+++ b/toolkit/mozapps/update/src/updater/updater.cpp
@@ -1153,6 +1153,12 @@ ActionList::Append(Action *action)
 int
 ActionList::Prepare()
 {
+  // If the action list is empty then we should fail in order to signal that
+  // something has gone wrong. Otherwise we report success when nothing is
+  // actually done. See bug 327140.
+  if (mCount == 0)
+    return UNEXPECTED_ERROR;
+
   Action *a = mFirst;
   while (a) {
     int rv = a->Prepare();