diff --git a/src/matrix/e2ee/RoomEncryption.js b/src/matrix/e2ee/RoomEncryption.js
index 430d9af3..d9151a85 100644
--- a/src/matrix/e2ee/RoomEncryption.js
+++ b/src/matrix/e2ee/RoomEncryption.js
@@ -16,7 +16,7 @@ limitations under the License.
 
 import {MEGOLM_ALGORITHM, DecryptionSource} from "./common.js";
 import {groupEventsBySession} from "./megolm/decryption/utils";
-import {mergeMap} from "../../utils/mergeMap.js";
+import {mergeMap} from "../../utils/mergeMap";
 import {groupBy} from "../../utils/groupBy";
 import {makeTxnId} from "../common.js";
 
diff --git a/src/matrix/e2ee/megolm/decryption/DecryptionPreparation.js b/src/matrix/e2ee/megolm/decryption/DecryptionPreparation.js
index 02ee32df..618955bb 100644
--- a/src/matrix/e2ee/megolm/decryption/DecryptionPreparation.js
+++ b/src/matrix/e2ee/megolm/decryption/DecryptionPreparation.js
@@ -15,7 +15,7 @@ limitations under the License.
 */
 
 import {DecryptionChanges} from "./DecryptionChanges.js";
-import {mergeMap} from "../../../../utils/mergeMap.js";
+import {mergeMap} from "../../../../utils/mergeMap";
 
 /**
  * Class that contains all the state loaded from storage to decrypt the given events
diff --git a/src/utils/mergeMap.js b/src/utils/mergeMap.ts
similarity index 93%
rename from src/utils/mergeMap.js
rename to src/utils/mergeMap.ts
index a0aed207..21ad0086 100644
--- a/src/utils/mergeMap.js
+++ b/src/utils/mergeMap.ts
@@ -14,7 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-export function mergeMap(src, dst) {
+export function mergeMap<K, V>(src: Map<K, V> | undefined, dst: Map<K, V>) {
     if (src) {
         for (const [key, value] of src.entries()) {
             dst.set(key, value);