mtext unicode failure

Readers,

How to solve this unicode input error please?

postscript("~/tmp/test.eps",width=100/25.4,height=100/25.4,horizontal=FALSE,onefile=TRUE,paper="special")
testx<-seq(1:5)
testy<-seq(1:5)
plot(testy~testx)
mtext('text (O?)\n more text',side=3,line=1)

Warning messages:
1: In mtext("text (O?)\n more text", side = 3, line = 1) :
  conversion failure on 'text (O?)' in 'mbcsToSbcs': dot substituted for <e2>
2: In mtext("text (O?)\n more text", side = 3, line = 1) :
  conversion failure on 'text (O?)' in 'mbcsToSbcs': dot substituted for <82>
3: In mtext("text (O?)\n more text", side = 3, line = 1) :
  conversion failure on 'text (O?)' in 'mbcsToSbcs': dot substituted for <82>
4: In mtext("text (O?)\n more text", side = 3, line = 1) :
  conversion failure on 'text (O?)' in 'mbcsToSbcs': dot substituted for <e2>
5: In mtext("text (O?)\n more text", side = 3, line = 1) :
  conversion failure on 'text (O?)' in 'mbcsToSbcs': dot substituted for <82>
6: In mtext("text (O?)\n more text", side = 3, line = 1) :
  conversion failure on 'text (O?)' in 'mbcsToSbcs': dot substituted for <82>

If the command to print to postscript is omitted, a graph is viewed
successfully, with the title text:

text(O?)
more text

Thanks.

--
r2151