diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index 54343bc30a420..5d9a140fec49e 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -3442,9 +3442,7 @@ nsDocShell::MaybeCreateInitialClientSource(nsIPrincipal* aPrincipal)
     ClientManager::CreateSource(ClientType::Window,
                                 win->EventTargetFor(TaskCategory::Other),
                                 principal);
-  if (NS_WARN_IF(!mInitialClientSource)) {
-    return;
-  }
+  MOZ_DIAGNOSTIC_ASSERT(mInitialClientSource);
 
   // Mark the initial client as execution ready, but owned by the docshell.
   // If the client is actually used this will cause ClientSource to force
diff --git a/dom/base/nsGlobalWindowInner.cpp b/dom/base/nsGlobalWindowInner.cpp
index 70e75ca1f8040..5280ea70ba8a0 100644
--- a/dom/base/nsGlobalWindowInner.cpp
+++ b/dom/base/nsGlobalWindowInner.cpp
@@ -1796,9 +1796,7 @@ nsGlobalWindowInner::EnsureClientSource()
     mClientSource = ClientManager::CreateSource(ClientType::Window,
                                                 EventTargetFor(TaskCategory::Other),
                                                 mDoc->NodePrincipal());
-    if (NS_WARN_IF(!mClientSource)) {
-      return NS_ERROR_FAILURE;
-    }
+    MOZ_DIAGNOSTIC_ASSERT(mClientSource);
     newClientSource = true;
   }
 
@@ -1828,9 +1826,7 @@ nsGlobalWindowInner::EnsureClientSource()
         ClientManager::CreateSource(ClientType::Window,
                                     EventTargetFor(TaskCategory::Other),
                                     mDoc->NodePrincipal());
-      if (NS_WARN_IF(!mClientSource)) {
-        return NS_ERROR_FAILURE;
-      }
+      MOZ_DIAGNOSTIC_ASSERT(mClientSource);
       newClientSource = true;
     }
   }
diff --git a/dom/clients/manager/ClientChannelHelper.cpp b/dom/clients/manager/ClientChannelHelper.cpp
index c1dd30b57c966..8d822766777ee 100644
--- a/dom/clients/manager/ClientChannelHelper.cpp
+++ b/dom/clients/manager/ClientChannelHelper.cpp
@@ -120,6 +120,7 @@ class ClientChannelHelper final : public nsIInterfaceRequestor
       reservedClient.reset();
       reservedClient = ClientManager::CreateSource(ClientType::Window,
                                                    mEventTarget, principal);
+      MOZ_DIAGNOSTIC_ASSERT(reservedClient);
 
       newLoadInfo->GiveReservedClientSource(Move(reservedClient));
     }
@@ -228,7 +229,7 @@ AddClientChannelHelper(nsIChannel* aChannel,
     reservedClient = ClientManager::CreateSource(ClientType::Window,
                                                  aEventTarget,
                                                  channelPrincipal);
-    NS_ENSURE_TRUE(reservedClient, NS_ERROR_FAILURE);
+    MOZ_DIAGNOSTIC_ASSERT(reservedClient);
   }
 
   RefPtr<ClientChannelHelper> helper =
diff --git a/dom/workers/ServiceWorkerManager.cpp b/dom/workers/ServiceWorkerManager.cpp
index 391b7e678b243..aee4819911909 100644
--- a/dom/workers/ServiceWorkerManager.cpp
+++ b/dom/workers/ServiceWorkerManager.cpp
@@ -2376,9 +2376,7 @@ ServiceWorkerManager::StartControllingADocument(ServiceWorkerRegistrationInfo* a
       RefPtr<ClientHandle> clientHandle =
         ClientManager::CreateHandle(clientInfo.ref(),
                                     SystemGroup::EventTargetFor(TaskCategory::Other));
-      if (clientHandle) {
-        ref = Move(clientHandle->Control(activeWorker->Descriptor()));
-      }
+      ref = Move(clientHandle->Control(activeWorker->Descriptor()));
     }
   }
 
@@ -2724,9 +2722,7 @@ ServiceWorkerManager::DispatchFetchEvent(const OriginAttributes& aOriginAttribut
         RefPtr<ClientHandle> clientHandle =
           ClientManager::CreateHandle(clientInfo.ref(),
                                       SystemGroup::EventTargetFor(TaskCategory::Other));
-        if (clientHandle) {
-          clientHandle->Control(serviceWorker->Descriptor());
-        }
+        clientHandle->Control(serviceWorker->Descriptor());
       }
 
       // But we also note the reserved state on the LoadInfo.  This allows the
diff --git a/dom/workers/WorkerPrivate.cpp b/dom/workers/WorkerPrivate.cpp
index 98fb0f2e3a0a1..f40f9913c9cdc 100644
--- a/dom/workers/WorkerPrivate.cpp
+++ b/dom/workers/WorkerPrivate.cpp
@@ -5332,9 +5332,7 @@ WorkerPrivate::EnsureClientSource()
 
   mClientSource = ClientManager::CreateSource(type, mWorkerHybridEventTarget,
                                               GetPrincipalInfo());
-  if (!mClientSource) {
-    return false;
-  }
+  MOZ_DIAGNOSTIC_ASSERT(mClientSource);
 
   if (mFrozen) {
     mClientSource->Freeze();