diff --git a/clientapi/routing/register.go b/clientapi/routing/register.go
index ff6a0900e..be2b192b2 100644
--- a/clientapi/routing/register.go
+++ b/clientapi/routing/register.go
@@ -31,6 +31,8 @@ import (
 	"time"
 
 	"github.com/matrix-org/dendrite/internal"
+	internalHTTPUtil "github.com/matrix-org/dendrite/internal/httputil"
+	"github.com/matrix-org/dendrite/internal/sqlutil"
 	"github.com/tidwall/gjson"
 
 	"github.com/matrix-org/dendrite/internal/eventutil"
@@ -859,6 +861,16 @@ func completeRegistration(
 				JSON: jsonerror.UserInUse("Desired user ID is already taken."),
 			}
 		}
+		switch e := err.(type) {
+		case internalHTTPUtil.InternalAPIError:
+			conflictErr := &userapi.ErrorConflict{Message: sqlutil.ErrUserExists.Error()}
+			if e.Message == conflictErr.Error() {
+				return util.JSONResponse{
+					Code: http.StatusBadRequest,
+					JSON: jsonerror.UserInUse("Desired user ID is already taken."),
+				}
+			}
+		}
 		return util.JSONResponse{
 			Code: http.StatusInternalServerError,
 			JSON: jsonerror.Unknown("failed to create account: " + err.Error()),