Why is Mask Editor self-modification checkbox disabled?

I have been trying to work through the Create a Custom Block example/tutorial, using MATLAB/Simulink R2016b. In the second part, on creating a dialog to configure the custom block by creating a mask, when I reach the third pane ("Initialization") of the Mask Editor, I am supposed to mark the "Allow library block to modify its contents" checkbox. But the checkbox is disabled (i.e., greyed out). Why is the checkbox disabled? What can I do about this? Thanks.

Answer 1

[Himanshu Tripathi]

This check box is enabled only if the masked block resides in a library.

Make sure you are putting your masked block in a library.