cannot redirect output expresso with coverage

[nalply]

$ expresso
... coverage output ...

but if I want to redirect the coverage report to coverage.txt, the file contains only the usual expresso report that all tests worked fine:

$ expresso > coverage.txt 2>&1
$ cat coverage.txt

100% 11 tests

Consider for example:

$ node -v; node -e 'process.stdout.write("test\n")' > out; cat out
v0.5.10
test