diff --git a/keyserver/internal/internal.go b/keyserver/internal/internal.go
index 06fc4987..d2ea2093 100644
--- a/keyserver/internal/internal.go
+++ b/keyserver/internal/internal.go
@@ -250,6 +250,7 @@ func (a *KeyInternalAPI) PerformMarkAsStaleIfNeeded(ctx context.Context, req *ap
 
 // nolint:gocyclo
 func (a *KeyInternalAPI) QueryKeys(ctx context.Context, req *api.QueryKeysRequest, res *api.QueryKeysResponse) error {
+	var respMu sync.Mutex
 	res.DeviceKeys = make(map[string]map[string]json.RawMessage)
 	res.MasterKeys = make(map[string]gomatrixserverlib.CrossSigningKey)
 	res.SelfSigningKeys = make(map[string]gomatrixserverlib.CrossSigningKey)
@@ -329,7 +330,7 @@ func (a *KeyInternalAPI) QueryKeys(ctx context.Context, req *api.QueryKeysReques
 	}
 
 	// attempt to satisfy key queries from the local database first as we should get device updates pushed to us
-	domainToDeviceKeys = a.remoteKeysFromDatabase(ctx, res, domainToDeviceKeys)
+	domainToDeviceKeys = a.remoteKeysFromDatabase(ctx, res, &respMu, domainToDeviceKeys)
 	if len(domainToDeviceKeys) > 0 || len(domainToCrossSigningKeys) > 0 {
 		// perform key queries for remote devices
 		a.queryRemoteKeys(ctx, req.Timeout, res, domainToDeviceKeys, domainToCrossSigningKeys)
@@ -407,7 +408,7 @@ func (a *KeyInternalAPI) QueryKeys(ctx context.Context, req *api.QueryKeysReques
 }
 
 func (a *KeyInternalAPI) remoteKeysFromDatabase(
-	ctx context.Context, res *api.QueryKeysResponse, domainToDeviceKeys map[string]map[string][]string,
+	ctx context.Context, res *api.QueryKeysResponse, respMu *sync.Mutex, domainToDeviceKeys map[string]map[string][]string,
 ) map[string]map[string][]string {
 	fetchRemote := make(map[string]map[string][]string)
 	for domain, userToDeviceMap := range domainToDeviceKeys {
@@ -415,7 +416,7 @@ func (a *KeyInternalAPI) remoteKeysFromDatabase(
 			// we can't safely return keys from the db when all devices are requested as we don't
 			// know if one has just been added.
 			if len(deviceIDs) > 0 {
-				err := a.populateResponseWithDeviceKeysFromDatabase(ctx, res, userID, deviceIDs)
+				err := a.populateResponseWithDeviceKeysFromDatabase(ctx, res, respMu, userID, deviceIDs)
 				if err == nil {
 					continue
 				}
@@ -542,7 +543,7 @@ func (a *KeyInternalAPI) queryRemoteKeysOnServer(
 		// refresh entries from DB: unlike remoteKeysFromDatabase we know we previously had no device info for this
 		// user so the fact that we're populating all devices here isn't a problem so long as we have devices.
 		respMu.Lock()
-		err = a.populateResponseWithDeviceKeysFromDatabase(ctx, res, userID, nil)
+		err = a.populateResponseWithDeviceKeysFromDatabase(ctx, res, respMu, userID, nil)
 		respMu.Unlock()
 		if err != nil {
 			logrus.WithFields(logrus.Fields{
@@ -573,7 +574,7 @@ func (a *KeyInternalAPI) queryRemoteKeysOnServer(
 	// inspecting the failures map though so they can know it's a cached response.
 	for userID, dkeys := range devKeys {
 		// drop the error as it's already a failure at this point
-		_ = a.populateResponseWithDeviceKeysFromDatabase(ctx, res, userID, dkeys)
+		_ = a.populateResponseWithDeviceKeysFromDatabase(ctx, res, respMu, userID, dkeys)
 	}
 
 	// Sytest expects no failures, if we still could retrieve keys, e.g. from local cache
@@ -585,7 +586,7 @@ func (a *KeyInternalAPI) queryRemoteKeysOnServer(
 }
 
 func (a *KeyInternalAPI) populateResponseWithDeviceKeysFromDatabase(
-	ctx context.Context, res *api.QueryKeysResponse, userID string, deviceIDs []string,
+	ctx context.Context, res *api.QueryKeysResponse, respMu *sync.Mutex, userID string, deviceIDs []string,
 ) error {
 	keys, err := a.DB.DeviceKeysForUser(ctx, userID, deviceIDs, false)
 	// if we can't query the db or there are fewer keys than requested, fetch from remote.
@@ -598,9 +599,11 @@ func (a *KeyInternalAPI) populateResponseWithDeviceKeysFromDatabase(
 	if len(deviceIDs) == 0 && len(keys) == 0 {
 		return fmt.Errorf("DeviceKeysForUser %s returned no keys but wanted all keys, falling back to remote", userID)
 	}
+	respMu.Lock()
 	if res.DeviceKeys[userID] == nil {
 		res.DeviceKeys[userID] = make(map[string]json.RawMessage)
 	}
+	respMu.Unlock()
 
 	for _, key := range keys {
 		if len(key.KeyJSON) == 0 {
@@ -610,7 +613,9 @@ func (a *KeyInternalAPI) populateResponseWithDeviceKeysFromDatabase(
 		key.KeyJSON, _ = sjson.SetBytes(key.KeyJSON, "unsigned", struct {
 			DisplayName string `json:"device_display_name,omitempty"`
 		}{key.DisplayName})
+		respMu.Lock()
 		res.DeviceKeys[userID][key.DeviceID] = key.KeyJSON
+		respMu.Unlock()
 	}
 	return nil
 }