--- a/netwerk/base/nsUDPSocket.cpp
+++ b/netwerk/base/nsUDPSocket.cpp
@@ -666,16 +666,20 @@ fail:
 
 NS_IMETHODIMP
 nsUDPSocket::Connect(const NetAddr *aAddr)
 {
   UDPSOCKET_LOG(("nsUDPSocket::Connect [this=%p]\n", this));
 
   NS_ENSURE_ARG(aAddr);
 
+  if (NS_WARN_IF(!mFD)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
   bool onSTSThread = false;
   mSts->IsOnCurrentThread(&onSTSThread);
   NS_ASSERTION(onSTSThread, "NOT ON STS THREAD");
   if (!onSTSThread) {
     return NS_ERROR_FAILURE;
   }
 
   PRNetAddr prAddr;