diff --git a/appservice/appservice.go b/appservice/appservice.go
index be5b30e2b..d78741983 100644
--- a/appservice/appservice.go
+++ b/appservice/appservice.go
@@ -16,6 +16,7 @@ package appservice
 
 import (
 	"context"
+	"errors"
 	"net/http"
 	"sync"
 	"time"
@@ -29,6 +30,7 @@ import (
 	"github.com/matrix-org/dendrite/appservice/workers"
 	"github.com/matrix-org/dendrite/clientapi/auth/storage/accounts"
 	"github.com/matrix-org/dendrite/clientapi/auth/storage/devices"
+	"github.com/matrix-org/dendrite/internal"
 	"github.com/matrix-org/dendrite/internal/basecomponent"
 	"github.com/matrix-org/dendrite/internal/config"
 	"github.com/matrix-org/dendrite/internal/transactions"
@@ -117,12 +119,12 @@ func generateAppServiceAccount(
 	ctx := context.Background()
 
 	// Create an account for the application service
-	acc, err := accountsDB.CreateAccount(ctx, as.SenderLocalpart, "", as.ID)
+	_, err := accountsDB.CreateAccount(ctx, as.SenderLocalpart, "", as.ID)
 	if err != nil {
+		if errors.Is(err, internal.ErrSQLUserExists) { // This account already exists
+			return nil
+		}
 		return err
-	} else if acc == nil {
-		// This account already exists
-		return nil
 	}
 
 	// Create a dummy device with a dummy token for the application service
diff --git a/clientapi/auth/storage/accounts/postgres/storage.go b/clientapi/auth/storage/accounts/postgres/storage.go
index 4a1832674..ba52ecfdc 100644
--- a/clientapi/auth/storage/accounts/postgres/storage.go
+++ b/clientapi/auth/storage/accounts/postgres/storage.go
@@ -164,7 +164,7 @@ func (d *Database) createAccount(
 	}
 	if err := d.profiles.insertProfile(ctx, txn, localpart); err != nil {
 		if internal.IsUniqueConstraintViolationErr(err) {
-			return nil, nil
+			return nil, internal.ErrSQLUserExists
 		}
 		return nil, err
 	}
diff --git a/clientapi/routing/register.go b/clientapi/routing/register.go
index 40a2862f8..e69d3bbfc 100644
--- a/clientapi/routing/register.go
+++ b/clientapi/routing/register.go
@@ -830,15 +830,16 @@ func completeRegistration(
 
 	acc, err := accountDB.CreateAccount(ctx, username, password, appserviceID)
 	if err != nil {
+		if errors.Is(err, internal.ErrSQLUserExists) { // user already exists
+			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()),
 		}
-	} else if acc == nil {
-		return util.JSONResponse{
-			Code: http.StatusBadRequest,
-			JSON: jsonerror.UserInUse("Desired user ID is already taken."),
-		}
 	}
 
 	// Increment prometheus counter for created users
diff --git a/internal/sql.go b/internal/sql.go
index d6a5a3086..e28a52a49 100644
--- a/internal/sql.go
+++ b/internal/sql.go
@@ -16,11 +16,15 @@ package internal
 
 import (
 	"database/sql"
+	"errors"
 	"fmt"
 	"runtime"
 	"time"
 )
 
+// ErrUserExists is returned if a username already exists in the database.
+var ErrSQLUserExists = errors.New("Username already exists")
+
 // A Transaction is something that can be committed or rolledback.
 type Transaction interface {
 	// Commit the transaction