Error When Attempting To Insert Record Using Stored Procedure

I am trying to save user data to a sql table but keep getting the following error;

Unable to cast object of type 'System.Boolean' to type 'System.Data.SqlClient.SqlParameter'.

I am using VWD Express Edition with .NET 2.0 and a SQL 2000 database. The code that generates the error is as follows;

Public Function chkUser(ByVal strUser As String) As String
dim MyConnection As SqlConnection
Dim MyCommand As SqlCommand
Dim ReturnString As String
Dim params As SqlParameter
Dim SelectCmd As String = "wm_CheckUser"
MyConnection = New SqlConnection(myConnectionString)
MyCommand = New SqlCommand(SelectCmd, MyConnection)
MyCommand.CommandType = CommandType.StoredProcedure

Try
params = MyCommand.Parameters.Add("@parUser", SqlDbType.VarChar, 100).Value = strUser
MyCommand.Connection.Open()
MyCommand.ExecuteNonQuery()
ReturnString = "Record Exists!"
Catch Exp As SqlException
ReturnString = ReturnError(Exp.Number, "Users", Exp.Message)
End Try
MyCommand.Connection.Close()
Return ReturnString

End Function

Basically, I'm traying to check to see if a user id already exists in the database before saving the data the user entered. If the email address entered by the user is already in the database I want a message to be shown to the user. If the email address does not exist then the data entered by the user is saved and the form goes to step two (2) of the user registration process.

Any help with this would be greately appreciated. I can't seem to see what is wrong here. Please someone help.

Thanks,

Jaime