plot(SOM) segfaults (PR#7157)

library(class)
plot(SOM)

segfaults in curve() at the line:

 y <- eval(expr, envir = list(x = x), enclos = parent.frame())

at least since R version 1.8.1.

Version:
 platform = i686-pc-linux-gnu
 arch = i686
 os = linux-gnu
 system = i686, linux-gnu
 status = Under development (unstable)
 major = 2
 minor = 0.0
 year = 2004
 month = 07
 day = 27
 language = R

Search Path:
 .GlobalEnv, package:class, package:methods, package:stats,
package:graphics, package:utils, Autoloads, package:base

Dr. David Meyer
Department of Information Systems

Vienna University of Economics and Business Administration
Augasse 2-6, A-1090 Wien, Austria, Europe
Fax: +43-1-313 36x746 
Tel: +43-1-313 36x4393
HP:  http://wi.wu-wien.ac.at/Wer_sind_wir/meyer/