I used UserAdmin.aspx in admin folder to add new user. After keyed in user information I clicked save button. The new user record was successfully saved. But exception occurs. Below is the exception message:

Column 'FirstName' does not belong to table Users. 
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.ArgumentException: Column 'FirstName' does not belong to table Users.

Source Error: 


Line 78:         Dim User As DataRow = Me.oUser.DataRow
Line 79: 
Line 80:         User(Me.oUser.FirstNameField) = Me.txtFirstName.Text
Line 81:         User(Me.oUser.LastNameField) = Me.txtLastName.Text
Line 82:         User(Me.oUser.UserIDField) = Me.txtUserId.Text

I didn't change any code in UserAdmin.aspx. May I know what is the possible reason causes this exception?