space chars are lost in overfull lines

In the following example I'm activating interword space chars. In
the first line this works fine. The second line is slightly overfull
and there the space chars are lost.

\pdfinterwordspaceon
\pdfcompresslevel=0
\pdfobjcompresslevel=0
\hsize=5cm
a a

a a xxxxxxxxxxxxxxxxxxxx.
a a
\bye

First line in the PDF:

Td [(a)]TJ/F51 9.9626 Tf( )Tj/F1 9.9626 Tf 8.302 0 Td [(a)]TJ

overfull line without space chars:
Td [(a)-222(a)-222(xxxxxxxxxxxxxxxxxxxx.)

next line of the paragraph
Td [(a)]TJ/F51 9.9626 Tf( )Tj/F1 9.9626 Tf 8.302 0 Td [(a)]


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