I am using dotConnect LINQ to MySQL and i have the following error. what would be the cause for this issue

annot convert parameter value of type 'System.String' to MySQL type 'MySqlType.TimeStamp'. 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.InvalidCastException: Cannot convert parameter value of type 'System.String' to MySQL type 'MySqlType.TimeStamp'.

Source Error:

Line 93: { Line 94: string loginLowered = login.ToLower();
Line 95: return context.ISVs.Where(u => u.Email == loginLowered).SingleOrDefault() == null; Line 96:
Line 97: }