Error In MS Stored Procedure

I am current creating a Oracle membership provider in dotnet based on the MS membership provider. One of the stored procedures that need converting is shown below. The variable UserId (highlighted blue) is declare and never assigned to but is used within the update query also highlighted in blue. If this is an oversight then I presume that this part of the stored procedure is never actually executed. Can somebody put an eye over this code to confirm.

Thanks.

ALTER PROCEDURE [dbo].[aspnet_Membership_GetUserByName]
@ApplicationName nvarchar(256),
@UserName nvarchar(256),
@CurrentTimeUtc datetime,
@UpdateLastActivity bit = 0
AS
BEGIN
DECLARE @UserId uniqueidentifier

IF (@UpdateLastActivity = 1)
BEGIN
SELECT TOP 1 m.Email, m.PasswordQuestion, m.Comment, m.IsApproved,
m.CreateDate, m.LastLoginDate, @CurrentTimeUtc, m.LastPasswordChangedDate,
u.UserId, m.IsLockedOut,m.LastLockoutDate
FROM dbo.aspnet_Applications a, dbo.aspnet_Users u, dbo.aspnet_Membership m
WHERE LOWER(@ApplicationName) = a.LoweredApplicationName AND
u.ApplicationId = a.ApplicationId AND
LOWER(@UserName) = u.LoweredUserName AND u.UserId = m.UserId

IF (@@ROWCOUNT = 0) -- Username not found
RETURN -1

UPDATE dbo.aspnet_Users
SET LastActivityDate = @CurrentTimeUtc
WHERE @UserId = UserId
END
ELSE
BEGIN
SELECT TOP 1 m.Email, m.PasswordQuestion, m.Comment, m.IsApproved,
m.CreateDate, m.LastLoginDate, u.LastActivityDate, m.LastPasswordChangedDate,
u.UserId, m.IsLockedOut,m.LastLockoutDate
FROM dbo.aspnet_Applications a, dbo.aspnet_Users u, dbo.aspnet_Membership m
WHERE LOWER(@ApplicationName) = a.LoweredApplicationName AND
u.ApplicationId = a.ApplicationId AND
LOWER(@UserName) = u.LoweredUserName AND u.UserId = m.UserId

IF (@@ROWCOUNT = 0) -- Username not found
RETURN -1
END

RETURN 0
END