NAs introduced by coercion warning?

I'm running a decision tree on a large dataset, and I'm getting multiple
instances of "NAs introduced by coercion" (> 50).  What does this mean?

--j

My guess would be you're trying to convert from character to numeric and 
are unable to do so. As in,

 > as.numeric("A")
[1] NA
Warning message:
NAs introduced by coercion
 > as.numeric("1")
[1] 1
 >

But without more information from you it's impossible to tell.

See the posting guide at

http://www.R-project.org/posting-guide.html

Regards,
Sundar