SPIEproceedings article error

[Alexandru Cabuz]

I get this error before the first citation in the text. 

Missing $ inserted.
 ...e \cite{Kosaka1998,Kosaka1999b,Baba2002b}.
                                                  
I've inserted a begin-math/end-math symbol since I think
you left one out. Proceed, with fingers crossed.

I think there is no $ missing, how do I tell it to not insert it ? Math mode 
has nothing to do with it, it's just a citation.

Thanks.

Alex.