--- a/netwerk/base/nsAsyncRedirectVerifyHelper.cpp
+++ b/netwerk/base/nsAsyncRedirectVerifyHelper.cpp
@@ -101,16 +101,21 @@ nsAsyncRedirectVerifyHelper::Init(nsICha
 
 NS_IMETHODIMP
 nsAsyncRedirectVerifyHelper::OnRedirectVerifyCallback(nsresult result)
 {
     LOG(("nsAsyncRedirectVerifyHelper::OnRedirectVerifyCallback() "
          "result=%x expectedCBs=%u mResult=%x",
          result, mExpectedCallbacks, mResult));
 
+    if (mExpectedCallbacks <= 0) {
+      NS_ERROR("OnRedirectVerifyCallback called more times than expected");
+      return NS_ERROR_UNEXPECTED;
+    }
+
     --mExpectedCallbacks;
 
     // If response indicates failure we may call back immediately
     if (NS_FAILED(result)) {
         // We chose to store the first failure-value (as opposed to the last)
         if (NS_SUCCEEDED(mResult))
             mResult = result;