Snap does not work with some layers

Snapping doesn't work with some layers due to a bug in Map. The issue is related to thematic rules and <null> values. Here is an example -  there is only one rule which is defined as

NOT IS_EXTERNAL IN (1)

IS_EXTERNAL contains <null> values.

To get snapping to work again you need to:

- add a second rule which deals with <null> values (e.g. : IS_EXTERNAL  NULL)
- set colours for the symbol used in the second rule to "none" (symbol will not be drawn)
- move rule on lowest position in style-editor.

Snapping restored....

...by fiddling around with thematic rules.

The exact circumstances of the causes of the issue are not known to me.
Map 2013, SP2