The rules file use the make variable $(_).
According to Policy 4.9, 'invoking either of
make -f debian/rules
<args...>' must result in identical behavior'.
One way to inadvertently violate this policy is to use the $_ variable.
If the rules file uses $(dir $(_)) to discover the directory containing the source package (presumably in order to implement the get-orig-source target), please replace it by $(dir $(firstword $(MAKEFILE_LIST))).