How to use xpath count in Fitnesse (Xebium)

Question

The headline basically says it all.

In Chrome and Firefox I can use the following expression:

count(//tbody/tr[contains(@id, 'myTable:0')])

It returns the number 2.

When I try to use this in Fitnesse (Xebium) it says that the execution of the command failed.

Is there a way to count the number of rows in my table in Fitnesse (Xebium), through xpath-expression.