Sass compiler error using Material Design Lite scss

I've just upgraded from 7.5 to 7.7.5 and now the Sass compiler is giving the following error:

[ERROR] com.vaadin.sass.internal.parser.ParseException: Only simple values are allowed as rect() parameters: ArgumentList [0 0 0 0]

which I've traced back to the mdl/material.scss import in my styles.scss.  The few uses of rect() in mdl are:

clip: rect(0 0 0 0);

which correlates with the error but I'm unclear why it would be causing an issue now.  The validation class RectFunctionGenerator.java hasn't been modified in the github repo since 2014, so it should be using the same logic unless 7.5 was deployed with an older Sass compiler version?

Modifying the MDL scss files to include 'px' fixes the issue, but this just opens future errors if MDL is updated and overrides the modifications.  Is this intended or a bug?

vaadin-server 7.5.10 requires at minimum vaadin-sass-compiler 0.9.12 (see https://mvnrepository.com/artifact/com.vaadin/vaadin-server/7.5.10)

vaadin-server 7.7.5 requires at minimum vaadin-sass-compiler 0.9.13 (see https://mvnrepository.com/artifact/com.vaadin/vaadin-server/7.7.5)

ok, sure, but according to this github. version 0.9.12 had a fix for rect(0 0 0 0) to pass the RectFunctionGenerator added on June 19, 2014.  so was this reverted in patch 13? That github doesn't appear to have any release notes for 13 so can't be 100% confident its the same compiler (though its under the vaadin account).  

This seems like a bug or missing documentation on how to compile.