[Groff] unquoted escape warning

[Anton Shterenlikht]

This line:

\h'15'

gives "unquoted escape" warning.
As far as I can see from the manual,
this horisontal motion is correct.
It is used to shift an equation right:

.EQ L \*[EQNXT]
\h'15'
E =
 { mu ( 3 lambda + 2 mu ) }
  over
   { lambda + mu }
~ ~~~
mark ;
~ ~~~
nu = lambda over { 2 ( lambda + mu ) }
.EN

What's the problem?

Thanks

Anton