Rush, OSX, won't accept cell array

I am mystified by this problem. Rush doesn't seem to work for me,
even though I think I am following the instructions correctly. Here
is an excerpt from the command line:

>> a={'clock'};

>> class(a)
ans =
cell

>> Rush(a)
Error in function : Usage error
First input argument must be either a cell array or a string.
??? Usage:
(null)
>>

Am I overlooking something simple?
Thanks for your help.