diff --git a/src/background/proxyService.test.ts b/src/background/proxyService.test.ts
index 2c4cd8c922..635ae360c4 100644
--- a/src/background/proxyService.test.ts
+++ b/src/background/proxyService.test.ts
@@ -176,13 +176,15 @@ describe("proxy service requests", () => {
     expect(data).toEqual({ foo: 42 });
   });
 
-  describe.each([[400], [401], [403], [500]])(
+  describe.each([[400], [401], [403], [405], [500]])(
     "remote status: %s",
     (statusCode) => {
       it("can proxy remote error", async () => {
+        const reason = "Bad request";
+
         axiosMock.onAny().reply(200, {
           json: {},
-          reason: "Bad request",
+          reason,
           status_code: statusCode,
         });
 
@@ -194,7 +196,7 @@ describe("proxy service requests", () => {
           const { status, statusText } = ((error as ContextError)
             .cause as AxiosError).response;
           expect(status).toEqual(statusCode);
-          expect(statusText).toEqual("Bad request");
+          expect(statusText).toEqual(reason);
         }
       });
     }
diff --git a/src/background/requests.ts b/src/background/requests.ts
index 17019fe5fe..ff4f38b8ba 100644
--- a/src/background/requests.ts
+++ b/src/background/requests.ts
@@ -109,11 +109,14 @@ const backgroundRequest = liftBackground(
   async (config: AxiosRequestConfig) => {
     try {
       return cleanResponse(await axios(config));
-    } catch (error) {
-      // Axios offers its own serialization method, but it doesn't include the response.
-      // By deleting toJSON, the serialize-error library will use its default serialization
+    } catch (error: unknown) {
+      if (isAxiosError(error)) {
+        // Axios offers its own serialization method, but it doesn't include the response.
+        // By deleting toJSON, the serialize-error library will use its default serialization
+        delete error.toJSON;
+      }
+
       console.trace("Error performing request from background page", { error });
-      delete error.toJSON;
       throw error;
     }
   }
diff --git a/src/contrib/google/bigquery/handlers.ts b/src/contrib/google/bigquery/handlers.ts
index 92a039459e..024f4f3aa9 100644
--- a/src/contrib/google/bigquery/handlers.ts
+++ b/src/contrib/google/bigquery/handlers.ts
@@ -41,9 +41,9 @@ async function ensureBigQuery(): Promise<void> {
   // https://github.com/google/google-api-javascript-client/blob/master/docs/reference.md
   try {
     await gapi.client.load("bigquery", "v2");
-  } catch (error) {
+  } catch (error: unknown) {
     console.debug("Error fetching BigQuery API definition", {
-      error: error.error,
+      error,
     });
     throw new Error("Error fetching BigQuery API definition");
   }
diff --git a/src/contrib/google/sheets/append.ts b/src/contrib/google/sheets/append.ts
index 336d108fbb..b6bbd6e708 100644
--- a/src/contrib/google/sheets/append.ts
+++ b/src/contrib/google/sheets/append.ts
@@ -15,14 +15,15 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-import { Effect } from "@/types";
+import { Effect, UnknownObject } from "@/types";
 import { BlockArg, BlockOptions, Schema } from "@/core";
 import { propertiesToSchema } from "@/validators/generic";
 import { isNullOrBlank } from "@/utils";
-import { unary } from "lodash";
+import { isPlainObject, unary } from "lodash";
 import { validateRegistryId } from "@/types/helpers";
 import { normalizeHeader } from "@/contrib/google/sheets/sheetsHelpers";
 import { sheets } from "@/background/messenger/api";
+import { getErrorMessage } from "@/errors";
 
 type CellValue = string | number | null;
 
@@ -103,8 +104,11 @@ export const GOOGLE_SHEETS_APPEND_ID = validateRegistryId(
   "@pixiebrix/google/sheets-append"
 );
 
-function isAuthError(error: { code: number }): boolean {
-  return [404, 401, 403].includes(error.code);
+function isAuthError(error: unknown): boolean {
+  return (
+    isPlainObject(error) &&
+    ([404, 401, 403] as unknown[]).includes((error as UnknownObject).code)
+  );
 }
 
 export class GoogleSheetsAppend extends Effect {
@@ -120,7 +124,15 @@ export class GoogleSheetsAppend extends Effect {
   inputSchema: Schema = APPEND_SCHEMA;
 
   async effect(
-    { spreadsheetId, tabName, rowValues: rawValues = {} }: BlockArg,
+    {
+      spreadsheetId,
+      tabName,
+      rowValues: rawValues = {},
+    }: BlockArg<{
+      spreadsheetId: string;
+      tabName: string;
+      rowValues: Record<string, CellValue> | RowValue[];
+    }>,
     { logger }: BlockOptions
   ): Promise<void> {
     const rowValues =
@@ -139,8 +151,10 @@ export class GoogleSheetsAppend extends Effect {
       console.debug(
         `Found headers for ${tabName}: ${currentHeaders.join(", ")}`
       );
-    } catch (error) {
-      logger.warn(`Error retrieving headers: ${error.toString()}`, error);
+    } catch (error: unknown) {
+      logger.warn(`Error retrieving headers: ${getErrorMessage(error)}`, {
+        error,
+      });
       if (isAuthError(error)) {
         throw error;
       }
diff --git a/src/devTools/Locator.tsx b/src/devTools/Locator.tsx
index 42280cfd29..d1a84c5983 100644
--- a/src/devTools/Locator.tsx
+++ b/src/devTools/Locator.tsx
@@ -25,22 +25,23 @@ import { useAsyncEffect } from "use-async-effect";
 import { isEmpty } from "lodash";
 import { thisTab } from "@/devTools/utils";
 import { detectFrameworks, searchWindow } from "@/contentScript/messenger/api";
+import { getErrorMessage } from "@/errors";
 
-function useSearchWindow(query: string) {
+function useSearchWindow(query: string): [unknown[] | null, unknown | null] {
   const { tabId } = browser.devtools.inspectedWindow;
-  const [results, setResults] = useState([]);
-  const [error, setError] = useState();
+  const [results, setResults] = useState<unknown[] | null>([]);
+  const [error, setError] = useState<unknown | null>();
 
   useAsyncEffect(
     async (isMounted) => {
       if (!query) return;
-      setError(undefined);
-      setResults(undefined);
+      setError(null);
+      setResults(null);
       try {
         const { results } = await searchWindow(thisTab, query);
         if (!isMounted()) return;
-        setResults(results as any);
-      } catch (error) {
+        setResults(results);
+      } catch (error: unknown) {
         if (!isMounted()) return;
         setError(error);
       }
@@ -92,7 +93,7 @@ const Locator: React.FunctionComponent = () => {
         />
       </InputGroup>
 
-      {searchError?.toString()}
+      {searchError && getErrorMessage(searchError)}
 
       {searchResults == null ? (
         <GridLoader />
diff --git a/tsconfig.json b/tsconfig.json
index 966e38213d..b5e5725c94 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -10,6 +10,7 @@
     "resolveJsonModule": true,
     "baseUrl": ".",
     "skipLibCheck": false,
+    "useUnknownInCatchVariables": true,
 
     // TODO: Drop these lines to make TS stricter https://github.com/pixiebrix/pixiebrix-extension/issues/775
     "strictNullChecks": false,
@@ -19,7 +20,6 @@
     "noImplicitReturns": false,
     "noUnusedParameters": false,
     "useDefineForClassFields": false,
-    "useUnknownInCatchVariables": false, // https://github.com/pixiebrix/pixiebrix-extension/issues/779
     "paths": {
       "@/*": ["src/*"],
       "@img/*": ["img/*"],