missing space chars at font switches

If one changes the font there are no space chars at the border. E.g.
with

\pdfcompresslevel0
\pdfobjcompresslevel0
\font\test=cmss10 
\pdfinterwordspaceon

text text {\test cmss cmss} text text

\bye

there is a space char between "text text" and "cmss cmss":

   [(text)]TJ/F51 9.9626 Tf( )Tj/F1 9.9626 Tf 20.756 0 Td [(text)]                            
   [(cmss)]TJ/F51 9.9626 Tf( )Tj/F20 9.9626 Tf 23.302 0 Td [(cmss)]

But nothing between "text cmss" and "cmss text"

   [(text)]TJ/F20 9.9626 Tf 20.755 0 Td [(cmss)] 
   [(cmss)]TJ/F1 9.9626 Tf 23.301 0 Td [(text)]

One can insert the missing chars manually with \pdffakespace but
perhaps an automatic solution is possible?

-- 
Ulrike Fischer 
http://www.troubleshooting-tex.de/