warning: Ignoring grob for slur. avoid-slur not set?

[Michael Kiermaier]

{c'( d'^\flageolet)}

gives the warning:

Ignoring grob for slur. avoid-slur not set?

I do not understand the meaning of the warning, since I don't know the meaning 
of the word "grob" (neither does dict.leo.org).

Anyway, the result looks fine, but how do I get rid of this annoying warning?

Thanks,

~michael