Contourf and NaNs (how to make white regions transparent)

Hello, I'm trying to make a contourf plot but certain areas of the data array have NaNs (only in the data matrix, the x and y meshgrid matrices are full). I'd like these NaNs to be transparent, and they are for NaNs on the boundary of the rectangle. But, contiguous NaN regions inside the data matrix are white instead of transparent. How can I make these transparent? Below is an example. Thanks,

-Justin

[Walter Roberson]

Tricky.