Data types in Slim are simple. They can only be Strings and Lists, though it is entirely possible for fixtures to actually get integers.
A list looks like this:
[1, 2, 0.5]
Note the brackets. Also, note the space between the comas and the following value. This is important, as the comparaison will actually be made between the exact string specified in the table, and the String representation of the List returned by the fixture. Hence, the format in toString() is important.