Keyboard cannot display when using UIAlertView on top of a modal view

Question

1.;

[m_splitViewController presentModalViewController:m_ModalViewController animated:NO];`

2.;

UIAlertView* alert = [[UIAlertView alloc] initWithTitle:@"login" message:strUserName
                       delegate: self cancelButtonTitle:@"Cancel"
                       otherButtonTitles:NSLocalizedString: @"Yes"];

[alert addTextFieldWithValue: @"" label:@""];

...

No keyboard appears. Pls help. Thanks.