diff --git a/spec/integ/crypto/crypto.spec.ts b/spec/integ/crypto/crypto.spec.ts
index 96d809aea9..af1142dfe0 100644
--- a/spec/integ/crypto/crypto.spec.ts
+++ b/spec/integ/crypto/crypto.spec.ts
@@ -3142,7 +3142,7 @@ describe.each(Object.entries(CRYPTO_BACKENDS))("crypto (%s)", (backend: string,
                     fetchMock.put(
                         "path:/_matrix/client/v3/room_keys/keys",
                         (url, request) => {
-                            const uploadPayload: KeyBackup = JSON.parse(request.body?.toString() ?? "{}");
+                            const uploadPayload: KeyBackup = JSON.parse((request.body as string) ?? "{}");
                             resolve(uploadPayload);
                             return {
                                 status: 200,
@@ -3209,7 +3209,7 @@ describe.each(Object.entries(CRYPTO_BACKENDS))("crypto (%s)", (backend: string,
                 fetchMock.post(
                     "path:/_matrix/client/v3/room_keys/version",
                     (url, request) => {
-                        const backupData: KeyBackupInfo = JSON.parse(request.body?.toString() ?? "{}");
+                        const backupData: KeyBackupInfo = JSON.parse((request.body as string) ?? "{}");
                         backupData.version = newVersion;
                         backupData.count = 0;
                         backupData.etag = "zer";
diff --git a/spec/integ/crypto/megolm-backup.spec.ts b/spec/integ/crypto/megolm-backup.spec.ts
index 05b5ff27b6..eff0ff567e 100644
--- a/spec/integ/crypto/megolm-backup.spec.ts
+++ b/spec/integ/crypto/megolm-backup.spec.ts
@@ -91,7 +91,7 @@ function mockUploadEmitter(
                     },
                 };
             }
-            const uploadPayload: KeyBackup = JSON.parse(request.body?.toString() ?? "{}");
+            const uploadPayload: KeyBackup = JSON.parse((request.body as string) ?? "{}");
             let count = 0;
             for (const [roomId, value] of Object.entries(uploadPayload.rooms)) {
                 for (const sessionId of Object.keys(value.sessions)) {
diff --git a/spec/test-utils/mockEndpoints.ts b/spec/test-utils/mockEndpoints.ts
index 988d6f13b6..7c16583885 100644
--- a/spec/test-utils/mockEndpoints.ts
+++ b/spec/test-utils/mockEndpoints.ts
@@ -88,7 +88,7 @@ export function mockSetupMegolmBackupRequests(backupVersion: string): void {
     });
 
     fetchMock.post("path:/_matrix/client/v3/room_keys/version", (url, request) => {
-        const backupData: KeyBackupInfo = JSON.parse(request.body?.toString() ?? "{}");
+        const backupData: KeyBackupInfo = JSON.parse((request.body as string) ?? "{}");
         backupData.version = backupVersion;
         backupData.count = 0;
         backupData.etag = "zer";
diff --git a/src/client.ts b/src/client.ts
index 2de41f2ca2..54f5309428 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -5065,7 +5065,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
             if (this.canSupport.get(Feature.RelationBasedRedactions) === ServerSupport.Unsupported) {
                 throw new Error(
                     "Server does not support relation based redactions " +
-                        `roomId ${roomId} eventId ${eventId} txnId: ${txnId} threadId ${threadId}`,
+                        `roomId ${roomId} eventId ${eventId} txnId: ${txnId as string} threadId ${threadId}`,
                 );
             }
 
diff --git a/src/crypto/store/indexeddb-crypto-store-backend.ts b/src/crypto/store/indexeddb-crypto-store-backend.ts
index 55a3d5aaff..6705ce0ed1 100644
--- a/src/crypto/store/indexeddb-crypto-store-backend.ts
+++ b/src/crypto/store/indexeddb-crypto-store-backend.ts
@@ -776,7 +776,7 @@ export class Backend implements CryptoStore {
                 ev.preventDefault();
                 logger.log("Ignoring duplicate inbound group session: " + senderCurve25519Key + " / " + sessionId);
             } else {
-                abortWithException(txn, new Error("Failed to add inbound group session: " + addReq.error));
+                abortWithException(txn, new Error("Failed to add inbound group session: " + addReq.error?.name));
             }
         };
     }
diff --git a/src/store/indexeddb-local-backend.ts b/src/store/indexeddb-local-backend.ts
index 0367380386..04a58dd410 100644
--- a/src/store/indexeddb-local-backend.ts
+++ b/src/store/indexeddb-local-backend.ts
@@ -71,7 +71,7 @@ function selectQuery<T>(
     return new Promise((resolve, reject) => {
         const results: T[] = [];
         query.onerror = (): void => {
-            reject(new Error("Query failed: " + query.error));
+            reject(new Error("Query failed: " + query.error?.name));
         };
         // collect results
         query.onsuccess = (): void => {
@@ -360,7 +360,7 @@ export class LocalIndexedDBStoreBackend implements IIndexedDBBackend {
                 // in firefox, with indexedDB disabled, this fails with a
                 // DOMError. We treat this as non-fatal, so that we can still
                 // use the app.
-                logger.warn(`unable to delete js-sdk store indexeddb: ${req.error}`);
+                logger.warn(`unable to delete js-sdk store indexeddb: ${req.error?.name}`);
                 resolve();
             };