diff --git a/clientapi/routing/register.go b/clientapi/routing/register.go
index 2de7b2733..b67e68e19 100644
--- a/clientapi/routing/register.go
+++ b/clientapi/routing/register.go
@@ -362,7 +362,7 @@ func UsernameMatchesMultipleExclusiveNamespaces(
 	// Check namespaces and see if more than one match
 	matchCount := 0
 	for _, appservice := range cfg.Derived.ApplicationServices {
-		if appservice.IsInterestedInUserID(userID) {
+		if appservice.OwnsNamespaceCoveringUserId(userID) {
 			if matchCount++; matchCount > 1 {
 				return true
 			}
@@ -1000,7 +1000,7 @@ func RegisterAvailable(
 	// Check if this username is reserved by an application service
 	userID := userutil.MakeUserID(username, cfg.Matrix.ServerName)
 	for _, appservice := range cfg.Derived.ApplicationServices {
-		if appservice.IsInterestedInUserID(userID) {
+		if appservice.OwnsNamespaceCoveringUserId(userID) {
 			return util.JSONResponse{
 				Code: http.StatusBadRequest,
 				JSON: jsonerror.UserInUse("Desired user ID is reserved by an application service."),
diff --git a/common/config/appservice.go b/common/config/appservice.go
index 7a43d48fb..bf5f089b7 100644
--- a/common/config/appservice.go
+++ b/common/config/appservice.go
@@ -98,6 +98,22 @@ func (a *ApplicationService) IsInterestedInUserID(
 	return false
 }
 
+// OwnsNamespaceCoveringUserId returns a bool on whether an application service's
+// namespace is exclusive and includes the given user ID
+func (a *ApplicationService) OwnsNamespaceCoveringUserId(
+	userID string,
+) bool {
+	if namespaceSlice, ok := a.NamespaceMap["users"]; ok {
+		for _, namespace := range namespaceSlice {
+			if namespace.Exclusive && namespace.RegexpObject.MatchString(userID) {
+				return true
+			}
+		}
+	}
+
+	return false
+}
+
 // IsInterestedInRoomAlias returns a bool on whether an application service's
 // namespace includes the given room alias
 func (a *ApplicationService) IsInterestedInRoomAlias(