substitute() bug? (PR#10525)

The first four lines of code below work as normal.  The fifth thorows  
an error: "Error in paste(theta[1], "=", 5) : object "theta" not found"


x=rnorm(1000);
bob=density(x);
topp=5;
plot(bob,xlab="", ylab="",  
main=substitute(paste(theta[1],"=",topp),list(topp=topp)), type="l");
plot(bob$y~bob$x,xlab="", ylab="",  
main=substitute(paste(theta[1],"=",topp),list(topp=topp)), type="l");
  

Note that theta is not defined anywhere:

ls()
  [1] "bob"      "dline"    "dplot"    "dpt"      "less"     "lesseq"
  [7] "more"     "moreeq"   "peak"     "peakline" "peakplot" "pop"
[13] "topp"     "x"


Version:
  platform = i386-apple-darwin8.10.1
  arch = i386
  os = darwin8.10.1
  system = i386, darwin8.10.1
  status =
  major = 2
  minor = 6.1
  year = 2007
  month = 11
  day = 26
  svn rev = 43537
  language = R
  version.string = R version 2.6.1 (2007-11-26)

Locale:
C

Search Path:
  .GlobalEnv, package:stats, package:graphics, package:grDevices,  
package:utils, package:datasets, package:methods,