diff --git a/packages/app-check/src/client.test.ts b/packages/app-check/src/client.test.ts
index d14c4db48bc..1c9befaefe9 100644
--- a/packages/app-check/src/client.test.ts
+++ b/packages/app-check/src/client.test.ts
@@ -107,7 +107,7 @@ describe('client', () => {
     const firebaseError = ERROR_FACTORY.create(
       AppCheckError.FETCH_NETWORK_ERROR,
       {
-        originalErrorMessage: originalError.message
+        originalErrorMessage: (originalError as Error)?.message
       }
     );
 
@@ -164,7 +164,7 @@ describe('client', () => {
     const firebaseError = ERROR_FACTORY.create(
       AppCheckError.FETCH_PARSE_ERROR,
       {
-        originalErrorMessage: originalError.message
+        originalErrorMessage: (originalError as Error)?.message
       }
     );
 
@@ -178,7 +178,7 @@ describe('client', () => {
       expect(e).has.property('message', firebaseError.message);
       expect(e).has.nested.property(
         'customData.originalErrorMessage',
-        originalError.message
+        (originalError as Error)?.message
       );
     }
   });
diff --git a/packages/app-check/src/client.ts b/packages/app-check/src/client.ts
index 6892e93e105..1861383174d 100644
--- a/packages/app-check/src/client.ts
+++ b/packages/app-check/src/client.ts
@@ -67,7 +67,7 @@ export async function exchangeToken(
     response = await fetch(url, options);
   } catch (originalError) {
     throw ERROR_FACTORY.create(AppCheckError.FETCH_NETWORK_ERROR, {
-      originalErrorMessage: originalError.message
+      originalErrorMessage: (originalError as Error)?.message
     });
   }
 
@@ -83,7 +83,7 @@ export async function exchangeToken(
     responseBody = await response.json();
   } catch (originalError) {
     throw ERROR_FACTORY.create(AppCheckError.FETCH_PARSE_ERROR, {
-      originalErrorMessage: originalError.message
+      originalErrorMessage: (originalError as Error)?.message
     });
   }
 
diff --git a/packages/app-check/src/indexeddb.ts b/packages/app-check/src/indexeddb.ts
index 7c27437e934..4041a535904 100644
--- a/packages/app-check/src/indexeddb.ts
+++ b/packages/app-check/src/indexeddb.ts
@@ -63,7 +63,7 @@ function getDBPromise(): Promise<IDBDatabase> {
     } catch (e) {
       reject(
         ERROR_FACTORY.create(AppCheckError.STORAGE_OPEN, {
-          originalErrorMessage: e.message
+          originalErrorMessage: (e as Error)?.message
         })
       );
     }
diff --git a/packages/app/src/indexeddb.ts b/packages/app/src/indexeddb.ts
index bf49522faab..6e7c991cc37 100644
--- a/packages/app/src/indexeddb.ts
+++ b/packages/app/src/indexeddb.ts
@@ -66,7 +66,7 @@ export async function readHeartbeatsFromIndexedDB(
       .get(computeKey(app)) as Promise<HeartbeatsInIndexedDB | undefined>;
   } catch (e) {
     throw ERROR_FACTORY.create(AppError.STORAGE_GET, {
-      originalErrorMessage: e.message
+      originalErrorMessage: (e as Error)?.message
     });
   }
 }
@@ -83,7 +83,7 @@ export async function writeHeartbeatsToIndexedDB(
     return tx.done;
   } catch (e) {
     throw ERROR_FACTORY.create(AppError.STORAGE_WRITE, {
-      originalErrorMessage: e.message
+      originalErrorMessage: (e as Error)?.message
     });
   }
 }
diff --git a/packages/remote-config/src/client/rest_client.ts b/packages/remote-config/src/client/rest_client.ts
index fe8c0156242..87fdae3c3d6 100644
--- a/packages/remote-config/src/client/rest_client.ts
+++ b/packages/remote-config/src/client/rest_client.ts
@@ -120,11 +120,11 @@ export class RestClient implements RemoteConfigFetchClient {
       response = await fetchPromise;
     } catch (originalError) {
       let errorCode = ErrorCode.FETCH_NETWORK;
-      if (originalError.name === 'AbortError') {
+      if ((originalError as Error)?.name === 'AbortError') {
         errorCode = ErrorCode.FETCH_TIMEOUT;
       }
       throw ERROR_FACTORY.create(errorCode, {
-        originalErrorMessage: originalError.message
+        originalErrorMessage: (originalError as Error)?.message
       });
     }
 
@@ -144,7 +144,7 @@ export class RestClient implements RemoteConfigFetchClient {
         responseBody = await response.json();
       } catch (originalError) {
         throw ERROR_FACTORY.create(ErrorCode.FETCH_PARSE, {
-          originalErrorMessage: originalError.message
+          originalErrorMessage: (originalError as Error)?.message
         });
       }
       config = responseBody['entries'];
diff --git a/packages/remote-config/src/client/retrying_client.ts b/packages/remote-config/src/client/retrying_client.ts
index fe1737023df..874a37bd8f4 100644
--- a/packages/remote-config/src/client/retrying_client.ts
+++ b/packages/remote-config/src/client/retrying_client.ts
@@ -124,7 +124,7 @@ export class RetryingClient implements RemoteConfigFetchClient {
 
       return response;
     } catch (e) {
-      if (!isRetriableError(e)) {
+      if (!isRetriableError(e as Error)) {
         throw e;
       }
 
diff --git a/packages/remote-config/src/storage/storage.ts b/packages/remote-config/src/storage/storage.ts
index d49ed0ab2c5..baa7ab46b52 100644
--- a/packages/remote-config/src/storage/storage.ts
+++ b/packages/remote-config/src/storage/storage.ts
@@ -29,7 +29,7 @@ import { FirebaseError } from '@firebase/util';
 function toFirebaseError(event: Event, errorCode: ErrorCode): FirebaseError {
   const originalError = (event.target as IDBRequest).error || undefined;
   return ERROR_FACTORY.create(errorCode, {
-    originalErrorMessage: originalError && originalError.message
+    originalErrorMessage: originalError && (originalError as Error)?.message
   });
 }
 
@@ -101,7 +101,7 @@ export function openDatabase(): Promise<IDBDatabase> {
     } catch (error) {
       reject(
         ERROR_FACTORY.create(ErrorCode.STORAGE_OPEN, {
-          originalErrorMessage: error
+          originalErrorMessage: (error as Error)?.message
         })
       );
     }
@@ -203,7 +203,7 @@ export class Storage {
       } catch (e) {
         reject(
           ERROR_FACTORY.create(ErrorCode.STORAGE_GET, {
-            originalErrorMessage: e && e.message
+            originalErrorMessage: (e as Error)?.message
           })
         );
       }
@@ -230,7 +230,7 @@ export class Storage {
       } catch (e) {
         reject(
           ERROR_FACTORY.create(ErrorCode.STORAGE_SET, {
-            originalErrorMessage: e && e.message
+            originalErrorMessage: (e as Error)?.message
           })
         );
       }
@@ -254,7 +254,7 @@ export class Storage {
       } catch (e) {
         reject(
           ERROR_FACTORY.create(ErrorCode.STORAGE_DELETE, {
-            originalErrorMessage: e && e.message
+            originalErrorMessage: (e as Error)?.message
           })
         );
       }
diff --git a/packages/remote-config/test/client/rest_client.test.ts b/packages/remote-config/test/client/rest_client.test.ts
index 4436ea252e6..abd3b7130d4 100644
--- a/packages/remote-config/test/client/rest_client.test.ts
+++ b/packages/remote-config/test/client/rest_client.test.ts
@@ -128,7 +128,7 @@ describe('RestClient', () => {
       const fetchPromise = client.fetch(DEFAULT_REQUEST);
 
       const firebaseError = ERROR_FACTORY.create(ErrorCode.FETCH_NETWORK, {
-        originalErrorMessage: originalError.message
+        originalErrorMessage: (originalError as Error)?.message
       });
 
       await expect(fetchPromise)
diff --git a/packages/util/test/errors.test.ts b/packages/util/test/errors.test.ts
index 9593b73dbcc..6b3ff9e7c91 100644
--- a/packages/util/test/errors.test.ts
+++ b/packages/util/test/errors.test.ts
@@ -88,9 +88,9 @@ describe('FirebaseError', () => {
     try {
       throw e;
     } catch (error) {
-      assert.isDefined(error.stack);
+      assert.isDefined((error as Error).stack);
       // Multi-line match trick - .* does not match \n
-      assert.match(error.stack, /FirebaseError[\s\S]/);
+      assert.match((error as Error).stack!, /FirebaseError[\s\S]/);
     }
   });
 
@@ -100,7 +100,8 @@ describe('FirebaseError', () => {
       assert.ok(false);
     } catch (e) {
       assert.instanceOf(e, FirebaseError);
-      assert.match(e.stack, /dummy2[\s\S]*?dummy1/);
+      assert.isDefined((e as FirebaseError).stack);
+      assert.match((e as FirebaseError).stack!, /dummy2[\s\S]*?dummy1/);
     }
   });
 });