diff --git a/appservice/appservice.go b/appservice/appservice.go
index be5b30e2..68cf52e7 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.ErrUserExists) { // 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/interface.go b/clientapi/auth/storage/accounts/interface.go
index a860f809..4d1941a2 100644
--- a/clientapi/auth/storage/accounts/interface.go
+++ b/clientapi/auth/storage/accounts/interface.go
@@ -29,6 +29,9 @@ type Database interface {
 	GetProfileByLocalpart(ctx context.Context, localpart string) (*authtypes.Profile, error)
 	SetAvatarURL(ctx context.Context, localpart string, avatarURL string) error
 	SetDisplayName(ctx context.Context, localpart string, displayName string) error
+	// CreateAccount makes a new account with the given login name and password, and creates an empty profile
+	// for this account. If no password is supplied, the account will be a passwordless account. If the
+	// account already exists, it will return nil, ErrUserExists.
 	CreateAccount(ctx context.Context, localpart, plaintextPassword, appserviceID string) (*authtypes.Account, error)
 	CreateGuestAccount(ctx context.Context) (*authtypes.Account, error)
 	UpdateMemberships(ctx context.Context, eventsToAdd []gomatrixserverlib.Event, idsToRemove []string) error
diff --git a/clientapi/auth/storage/accounts/postgres/storage.go b/clientapi/auth/storage/accounts/postgres/storage.go
index 4a183267..4be5dca9 100644
--- a/clientapi/auth/storage/accounts/postgres/storage.go
+++ b/clientapi/auth/storage/accounts/postgres/storage.go
@@ -138,7 +138,7 @@ func (d *Database) CreateGuestAccount(ctx context.Context) (acc *authtypes.Accou
 
 // CreateAccount makes a new account with the given login name and password, and creates an empty profile
 // for this account. If no password is supplied, the account will be a passwordless account. If the
-// account already exists, it will return nil, nil.
+// account already exists, it will return nil, ErrUserExists.
 func (d *Database) CreateAccount(
 	ctx context.Context, localpart, plaintextPassword, appserviceID string,
 ) (acc *authtypes.Account, err error) {
@@ -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.ErrUserExists
 		}
 		return nil, err
 	}
diff --git a/clientapi/auth/storage/accounts/sqlite3/storage.go b/clientapi/auth/storage/accounts/sqlite3/storage.go
index 7dec8729..4b436708 100644
--- a/clientapi/auth/storage/accounts/sqlite3/storage.go
+++ b/clientapi/auth/storage/accounts/sqlite3/storage.go
@@ -27,8 +27,8 @@ import (
 	"github.com/matrix-org/gomatrixserverlib"
 	"golang.org/x/crypto/bcrypt"
 
-	// Import the postgres database driver.
-	_ "github.com/mattn/go-sqlite3"
+	// Import the sqlite3 database driver.
+	"github.com/mattn/go-sqlite3"
 )
 
 // Database represents an account database
@@ -148,7 +148,7 @@ func (d *Database) CreateGuestAccount(ctx context.Context) (acc *authtypes.Accou
 
 // CreateAccount makes a new account with the given login name and password, and creates an empty profile
 // for this account. If no password is supplied, the account will be a passwordless account. If the
-// account already exists, it will return nil, nil.
+// account already exists, it will return nil, ErrUserExists.
 func (d *Database) CreateAccount(
 	ctx context.Context, localpart, plaintextPassword, appserviceID string,
 ) (acc *authtypes.Account, err error) {
@@ -172,8 +172,8 @@ func (d *Database) createAccount(
 		}
 	}
 	if err := d.profiles.insertProfile(ctx, txn, localpart); err != nil {
-		if internal.IsUniqueConstraintViolationErr(err) {
-			return nil, nil
+		if errors.Is(err, sqlite3.ErrConstraint) {
+			return nil, internal.ErrUserExists
 		}
 		return nil, err
 	}
diff --git a/clientapi/routing/register.go b/clientapi/routing/register.go
index 40a2862f..d356db2c 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.ErrUserExists) { // 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/cmd/create-account/main.go b/cmd/create-account/main.go
index eca9b2fe..9c1e4593 100644
--- a/cmd/create-account/main.go
+++ b/cmd/create-account/main.go
@@ -69,13 +69,10 @@ func main() {
 		os.Exit(1)
 	}
 
-	account, err := accountDB.CreateAccount(context.Background(), *username, *password, "")
+	_, err = accountDB.CreateAccount(context.Background(), *username, *password, "")
 	if err != nil {
 		fmt.Println(err.Error())
 		os.Exit(1)
-	} else if account == nil {
-		fmt.Println("Username already exists")
-		os.Exit(1)
 	}
 
 	deviceDB, err := devices.NewDatabase(*database, nil, serverName)
diff --git a/internal/sql.go b/internal/sql.go
index 546954bd..e3c10afc 100644
--- a/internal/sql.go
+++ b/internal/sql.go
@@ -26,6 +26,9 @@ import (
 	"go.uber.org/atomic"
 )
 
+// ErrUserExists is returned if a username already exists in the database.
+var ErrUserExists = errors.New("Username already exists")
+
 // A Transaction is something that can be committed or rolledback.
 type Transaction interface {
 	// Commit the transaction