diff --git a/lib/ts-types/localForage.d.ts b/lib/ts-types/localForage.d.ts
index e5d42d6d..e8e6fc16 100644
--- a/lib/ts-types/localForage.d.ts
+++ b/lib/ts-types/localForage.d.ts
@@ -18,8 +18,8 @@ interface LocalForageOptions {
 }
 
 interface LocalForageDbMethods {
-    getItem<T>(key: string): Promise<T>;
-    getItem<T>(key: string, callback: (err: any, value: T) => void): void;
+    getItem<T>(key: string): Promise<T | null>;
+    getItem<T>(key: string, callback: (err: any, value: T | null) => void): void;
 
     setItem<T>(key: string, value: T): Promise<T>;
     setItem<T>(key: string, value: T, callback: (err: any, value: T) => void): void;
diff --git a/package-lock.json b/package-lock.json
index cfa9ad5e..a2cbc96d 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -705,9 +705,9 @@
       "integrity": "sha512-81oxVi/OY+LrzgrONX7ciD1wtvq24nb2M9iYixRiQG+1hrDrDRqFVWuQzF1fUexh3Sg/ol6eMAz1MOVYFouzUg=="
     },
     "node_modules/kaitai-struct-compiler": {
-      "version": "0.11.0-SNAPSHOT20231104.85023.e581718",
-      "resolved": "https://registry.npmjs.org/kaitai-struct-compiler/-/kaitai-struct-compiler-0.11.0-SNAPSHOT20231104.85023.e581718.tgz",
-      "integrity": "sha512-1i5jAfwSAxrB6HkItjdZwpOJqkcHEcoQdRhvFp1U3n5kSTtsuX/0yos+5D65GONcZkl3rf21IN4zwkSsjjMrOw=="
+      "version": "0.11.0-SNAPSHOT20231104.124722.3af7a08",
+      "resolved": "https://registry.npmjs.org/kaitai-struct-compiler/-/kaitai-struct-compiler-0.11.0-SNAPSHOT20231104.124722.3af7a08.tgz",
+      "integrity": "sha512-iZFRd7Md3tK8+WlD3WBmrIIPTEfmVWWMSNgfeiHOjDS8XAXZBvbvjdtkuMcUgJ1AMbZfozvKK4ecDy6x/scaig=="
     },
     "node_modules/lie": {
       "version": "3.1.1",
diff --git a/src/v1/KaitaiServices.ts b/src/v1/KaitaiServices.ts
index 7261b040..569c6a7e 100644
--- a/src/v1/KaitaiServices.ts
+++ b/src/v1/KaitaiServices.ts
@@ -57,8 +57,27 @@ class JsImporter implements IYamlImporter {
         }
 
         console.log(`import yaml: ${name}, mode: ${mode}, loadFn: ${loadFn}, root:`, this.rootFsItem);
-        let ksyContent = await fss[importedFsType].get(`${loadFn}.ksy`);
-        var ksyModel = <KsySchema.IKsyFile>YAML.parse(<string>ksyContent);
+        const fn = `${loadFn}.ksy`;
+        const sourceAppendix = mode === 'abs' ? 'kaitai.io' : 'local storage';
+        let ksyContent;
+        try {
+            ksyContent = await fss[importedFsType].get(`${loadFn}.ksy`);
+        } catch (e) {
+            const error = new Error(`failed to import spec ${fn} from ${sourceAppendix}${e.message ? ': ' + e.message : ''}`);
+
+            // The default implementation of the Error.prototype.toString() method gives
+            // "Error: {message}", see
+            // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/toString#description
+            // However, the error we're throwing here goes directly into the KSC code,
+            // which will add its own `error: ` prefix (as the severity of the problem),
+            // so the resulting message would contain `error: Error: ...`. By overriding
+            // toString() to omit the `Error: ` part, we can make the message a bit nicer.
+            error.toString = function () {
+                return this.message;
+            };
+            throw error;
+        }
+        const ksyModel = <KsySchema.IKsyFile>YAML.parse(<string>ksyContent);
         return ksyModel;
     }
 }
diff --git a/src/v1/app.files.ts b/src/v1/app.files.ts
index 548d8e16..d50e6852 100644
--- a/src/v1/app.files.ts
+++ b/src/v1/app.files.ts
@@ -67,7 +67,15 @@ class LocalStorageFs implements IFileSystem {
         return this.save();
     }
 
-    get(fn: string): Promise<string | ArrayBuffer> { return localforage.getItem<string|ArrayBuffer>(this.fileKey(fn)); }
+    get(fn: string): Promise<string | ArrayBuffer> {
+        return localforage.getItem<string | ArrayBuffer>(this.fileKey(fn))
+            .then(content => {
+                if (content === null) {
+                    throw new Error('file not found');
+                }
+                return content;
+            });
+    }
 
     put(fn: string, data: any): Promise<IFsItem> {
         return this.getRootNode().then(root => {
@@ -84,7 +92,25 @@ class KaitaiFs implements IFileSystem {
 
     get(fn: string): Promise<string|ArrayBuffer> {
         if (fn.toLowerCase().endsWith(".ksy"))
-            return Promise.resolve<string>(<any>$.ajax({ url: fn }));
+            return fetch(fn)
+                .then(response => {
+                    if (!response.ok) {
+                        let msg;
+                        if (response.status === 404) {
+                            msg = 'file not found';
+                        } else {
+                            const textAppendix = response.statusText ? ` (${response.statusText})` : '';
+                            msg = `server responded with HTTP status ${response.status}${textAppendix}`;
+                        }
+                        throw new Error(msg);
+                    }
+                    return response.text();
+                }, err => {
+                    if (err instanceof TypeError) {
+                        throw new Error(`cannot reach the server (message: ${err.message}), check your internet connection`);
+                    }
+                    throw err;
+                });
         else
             return downloadFile(fn);
     }
diff --git a/src/v1/app.layout.ts b/src/v1/app.layout.ts
index 445e18f3..570f8c9f 100644
--- a/src/v1/app.layout.ts
+++ b/src/v1/app.layout.ts
@@ -121,7 +121,8 @@ export class UI {
         this.layout.addEditor("genCodeDebugViewer", "javascript", false);
         this.layout.addComponent("hexViewer", () => {
             var hexViewer = new HexViewer("#hexViewer");
-            hexViewer.bytesPerLine = parseInt(localStorage.getItem("HexViewer.bytesPerLine")) || 16;
+            const stored = localStorage.getItem("HexViewer.bytesPerLine");
+            hexViewer.bytesPerLine = (stored !== null && parseInt(stored, 10)) || 16;
             return hexViewer;
         });
         this.layout.addComponent("errorWindow", cont => { cont.getElement().append($("<div />")); });
@@ -132,4 +133,4 @@ export class UI {
     }
 
     init() { this.layout.layout.init(); }
-}
\ No newline at end of file
+}
diff --git a/src/v1/app.ts b/src/v1/app.ts
index 6d7669bb..a24122e7 100644
--- a/src/v1/app.ts
+++ b/src/v1/app.ts
@@ -279,7 +279,8 @@ $(() => {
     app.formatReady = loadCachedFsItem(app.ksyFsItemName, "kaitai", "formats/archive/zip.ksy");
 
     app.inputReady.then(() => {
-        var storedSelection = JSON.parse(localStorage.getItem("selection"));
+        const value = localStorage.getItem("selection");
+        const storedSelection = value !== null ? JSON.parse(value) : null;
         if (storedSelection)
             app.ui.hexViewer.setSelection(storedSelection.start, storedSelection.end);
     });
diff --git a/tsconfig.json b/tsconfig.json
index a9b4ad52..b4fa3b73 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -10,6 +10,7 @@
     "allowSyntheticDefaultImports": true,
     "experimentalDecorators": true,
     "alwaysStrict": true,
+//    "strictNullChecks": true,
     "sourceMap": false,
     "target": "es2017",
     "outDir": "js/",