How to pass MySQL null parameter to DbCommand in corporate library

I am trying to pass null for the first parameter in the code below, but MySQL is complaining that

Incorrect number of arguments for PROCEDURE myProc; expected 2, got 1

When I manually call the procedure with the first argument as null it works, but when it EmptyAsNullStartsWith(employeeNumberText.Text)

returns null it complains.

Database db = DatabaseFactory.CreateDatabase(
  ConfigurationManager.AppSettings["dbType"]
);
DbCommand cmd = db.GetStoredProcCommand("staff_listforinquiry");
db.AddeParameter(
  cmd, 
  "in_employeeNumber", 
  DbType.String, 
  EmptyAsNullStartsWith(employeeNumberText.Text)
);
db.AddeParameter(
  cmd, 
  "in_name", 
  DbType.String, 
  EmptyAsNullContains(employeeNameText.Text)
);