I see you're new, check what rowcased said, note the quotes around the first value, that's what your function returned and the second value is the expected one.

first: don't post code without the spoiler tag, even if it doesn't work right

okay, now take a careful look at your ever-handy error message: '987654321' should equal 987654321

The max value an int can be 2^32. There is a possibility that an given int might result in a number after digit sorting that does not fit int range.
The return value of the function could be changed to long to cater to such cases. Thoughts??