Erroneous handling of "cd" in markdown bash syntax highlighting

In markdown containing bash code, "cd" is highlighted such that it looks like a comment. In the example below, I would expect "cd" to appear the same as "mkdir".

#!bash
# This is a comment
mkdir ~/repo
cd ~/repo

FWIW the resulting HTML span element corresponding to "cd" has the class "nb" (I haven't been able to work out which language feature this corresponds to, comments appear to have the class "c1").