diff --git a/service-worker.js b/service-worker.js
index 0658489..7a381a9 100644
--- a/service-worker.js
+++ b/service-worker.js
@@ -177,9 +177,11 @@ let decrementActiveFetches = (clientId) => {
         // (and new fetches will fire in a moment, because a CSS file just
         //  got fetched) or not
         self.clients.get(clientId).then((client)=>{
-            client.postMessage({
-                allFetched: true
-            })
+            if (client !== null) {
+                client.postMessage({
+                    allFetched: true
+                })
+            }
         })
         .then(()=>{
             self.log('service-worker', 'all-fetched message sent.')
@@ -255,8 +257,10 @@ let LibResilientResourceInfo = class {
             self.clients.get(clientId).then((client)=>{
                 // set the client
                 this.client = client
-                // Send a message to the client.
-                this.client.postMessage(this.values);
+                // Send a message to the client
+                if (this.client !== null) {
+                    this.client.postMessage(this.values);
+                }
             })
         }
     }
@@ -287,7 +291,7 @@ let LibResilientResourceInfo = class {
             })
         self.log('service-worker', msg)
         // send the message to the client
-        if (this.client && changed) {
+        if (this.client && changed && (this.client !== null)) {
             this.client.postMessage(this.values);
         }
     }
@@ -497,10 +501,12 @@ let getResourceThroughLibResilient = (request, clientId, useStashed=true, doStas
                     // inform!
                     self.log('service-worker', 'fetched version method or ETag differs from stashed for:', url)
                     self.clients.get(reqInfo.clientId).then((client)=>{
-                        client.postMessage({
-                            url: url,
-                            fetchedDiffers: true
-                        })
+                        if (client !== null) {
+                            client.postMessage({
+                                url: url,
+                                fetchedDiffers: true
+                            })
+                        }
                     })
                 }
             }
@@ -582,7 +588,7 @@ self.addEventListener('fetch', event => {
     // if event.resultingClientId is available, we need to use this
     // otherwise event.clientId is what we want
     // ref. https://developer.mozilla.org/en-US/docs/Web/API/FetchEvent/resultingClientId
-    var clientId = event.clientId
+    var clientId = (event.clientId !== null) ? event.clientId : 'unknown-client'
     if (event.resultingClientId) {
         clientId = event.resultingClientId
         // yeah, we seem to have to send the client their clientId
@@ -595,11 +601,13 @@ self.addEventListener('fetch', event => {
         self.clients
             .get(clientId)
             .then((client)=>{
-                client.postMessage({
-                    clientId: clientId,
-                    plugins: self.LibResilientPlugins.filter(p=>(!p.indirect)).map((p)=>{return p.name}),
-                    serviceWorker: 'COMMIT_UNKNOWN'
-                })
+                if (client !== null) {
+                    client.postMessage({
+                        clientId: clientId,
+                        plugins: self.LibResilientPlugins.filter(p=>(!p.indirect)).map((p)=>{return p.name}),
+                        serviceWorker: 'COMMIT_UNKNOWN'
+                    })
+                }
             })
     }