Unrecognized character in a font? (Substituting .notdef for space . . .)

[Lichen]

I'm getting this message while converting a PDF to a PNG:

    "Substituting .notdef for space in the font HHGMNB+BSISans,Bold"

The output is ugly -- spaces are turned into little boxes.  Is there
something I can do to make sure the "space" character is mapped in
this font?  Or would it be easier to have the person building the PDF
use a different font that is more easily reconizable?

Thanks in advance.

Tim in Oregon