cario.Context.TextExtents(aString) changes cario.Context.target type

[coops]

anyone konw why method
cario.Context.TextExtents(aString)
changes
cario.Context.target
from pdfSurface to imageSurface

i am using cario to generate pdf output, so the Context.target is a pdfSurface.
however, when I use the method cario.Context.TextExtents(aString) to output a string into the pdf file, some string make this method change the Context.target from pdfsurface to imageSurface and make my application crash.

anyone meet this problem before?
have any idea to solve it ?

thanks a lot