Exception in converting string to person or group type

Question

I want to insert the user account for each user in a list column. As the list column is of type Person or Group, I need to convert the string type value to the person or group type. I have used the following function to convert the string value to person or group type :

    public SPFieldUserValue convertToPersonorGroup(string userid,SPWeb web)
    {            
        if(userid != null)
        {
            SPUser requireduser = web.EnsureUser(userid);
            SPFieldUserValue uservalue = new SPFieldUserValue(web, requireduser.ID,requireduser.LoginName);
            return uservalue;
        }
        return null;
    }

This function works fine for most of the user account names but in one of the account name it throws "The specified user indusa\ezend could not be found." exception.

I have also checked in the User Profile Service and this user exists there.but still I get this exception. I am not able to figure out the cause of the error. Kindly help.

Thanks in advance.