use of \citet (with natbib numeric) gives "(author?)" error

[Matthias Hunstig]

Hello,

I am using LyX 2.0 and just switched the citation style to Natbib (numeric) 
because I want to use \citet from time to time.

However, whenever I use \citet, a bold *(author?)* appears in the PDF instead 
of the author name. The source code looks fine to me, for example: 
\citet[p.~223]{watson_2009}

Any idea what causes this?

Thanks

Matthias