Hi, running the following example code (taken from the docs) will prodice an error in my system (R 2.5.0, win): library(debug) mtrace(glm) # turns tracing on names( tracees) # "glm" check.for.tracees( "package:base") # "glm" glm(stupid.args) # voila le debugger> Error in all.levs[[j]] : subscript out of boundsIt happens with any function, I just posted one form the example. Thanks, -J -- Jose Quesada, PhD. http://www.andrew.cmu.edu/~jquesada