maptools package rejects some polygon shapefiles?

Greetings: 

Using maptools in the Win XP environment, trying to
display an ESRI Shapefile (created with ArcMAP GIS ver 9)
outline map of North America. read.shape throws error:
Error in read.shape: Not a valid shape type. 

read.sahpe does read (and plot displays) a point shapefile 
created on the same platform. 

Any hints on determining the problem with 'not valid' shapefiles
in R? 

Thanks, 
Rick R


Scientific Programmer / Quantitative Analyst
National Center for Ecological Analysis and Synthesis
University of California, Santa Barbara
805 892 2534
reeves at nceas.ucsb.edu