[groff] problem with umlaut in string

[Ulrich Lauther]

I use 

.ds BLD \f3\\$*\fP

and

this text is partly  \*[BLD in bold face]

to switch to bold face and back within a line of text.

This works fine as long as there is no german umlaut in the argument to BLD.

With an umlaut, I get

groff -Tps -k  -t -mom -P-pa4 - > /tmp/4451out.ps
<standard input>:219: missing `]'
<standard input>:219: invalid base glyph `u00F6\fPr]' in composite glyph name
<standard input>:219: warning: can't find special character `'

Seems, groff is confused by the '[' produced by preconv.

Is this a groff bug, or my fault?

Kind regards,

     ulrich