problems with pdf_place_image

Dear Friends:

    I´m using php-4.0 with pdflib 3.0 but i have the following problem:

    In the line:

        pdf_place_image($pdf,$im,50,728,0.21);

    i obtain the error message:

        Fatal error: Internal pdflib error: Scaling factor 0 for image
...

    It seem´s that the scale factor of the pdf_place_image can´t be a
real number, but in the documentation the
type of this scale factor is double.

    Any idea?.

    Thank for all.