--- a/netwerk/protocol/http/nsHttpChannel.cpp
+++ b/netwerk/protocol/http/nsHttpChannel.cpp
@@ -36,16 +36,17 @@
 #include "mozilla/TimeStamp.h"
 #include "nsError.h"
 #include "nsPrintfCString.h"
 #include "nsAlgorithm.h"
 #include "nsQueryObject.h"
 #include "GeckoProfiler.h"
 #include "nsIConsoleService.h"
 #include "mozilla/Attributes.h"
+#include "mozilla/DebugOnly.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/VisualEventTracer.h"
 #include "nsISSLSocketControl.h"
 #include "sslt.h"
 #include "nsContentUtils.h"
 #include "nsIClassOfService.h"
 #include "nsIPermissionManager.h"
 #include "nsIPrincipal.h"
@@ -2788,16 +2789,20 @@ nsHttpChannel::OpenCacheEntry(bool isHtt
     }
     if (PossiblyIntercepted()) {
         extension.Append(nsPrintfCString("u%lld", mInterceptionID));
     }
 
     // If this channel should be intercepted, we do not open a cache entry for this channel
     // until the interception process is complete and the consumer decides what to do with it.
     if (mInterceptCache == MAYBE_INTERCEPT) {
+        DebugOnly<bool> exists;
+        MOZ_ASSERT(NS_FAILED(cacheStorage->Exists(openURI, extension, &exists)) || !exists,
+                   "The entry must not exist in the cache before we create it here");
+
         nsCOMPtr<nsICacheEntry> entry;
         rv = cacheStorage->OpenTruncate(openURI, extension, getter_AddRefs(entry));
         NS_ENSURE_SUCCESS(rv, rv);
 
         nsCOMPtr<nsINetworkInterceptController> controller;
         GetCallback(controller);
 
         nsRefPtr<InterceptedChannelChrome> intercepted =