I'm running R 1.1.0 for Windows; the behaviour is the same in 1.0.1. # create a list containing an expression: no problem x _ list(NA, expression(1)) # Try to assign an expression to a list element, and get an error: x[[1]] _ expression(1) # Try again with a named list element: no problem! x _ list(a=NA,expression(1)) x$a _ expression(1) The error message is: Error in "[[<-"(*tmp*, 1, value = expression(1)) : incompatible types I think the error is in "[[<-", but that's a primitive, and I don't know how to track it down any further. Duncan Murdoch -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- r-devel mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html Send "info", "help", or "[un]subscribe" (in the "body", not the subject !) To: r-devel-request@stat.math.ethz.ch _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._