Renaming or aliasing NH macro causes stack limit error

Working with the ms macros, I'm getting a weird error whenever I try to rename
or alias the NH macro. Here is a minimal example:

	$ groff -ms >/dev/null <<EOF
	.rn NH FOO
	.FOO
	foo
	EOF
	troff:<standard input>:2: fatal error: input stack limit of 1000 levels
	exceeded (probable infinite loop)

	$ groff -ms >/dev/null <<EOF
	.als FOO NH
	.FOO
	foo
	EOF
	troff:<standard input>:2: fatal error: input stack limit of 1000 levels
	exceeded (probable infinite loop)

Any experts know why this happens? Is it expected, or a bug?
-- 
S.