[R] Error in pruning hclust object using maptree package

[Keith]

Dear R users,

Due to too many children in my clustering result, I would like to prune 
the tree-like hclust object into a certain groups. However, an error:

 > clip.clust: no data provided for hclust object

always shows up.

Firstly, I tried the example in the maptree package, and it worked well. 
Then, I tried to used the example in stats package to generate a hclust 
object and tried the clip.clust method again, and the error still 
occurred. Here is the example:

 > hc <- hclust(dist(USArrests), "ave")
 > pr <- clip.clust(hc, k=4)
Error in clip.clust(hc, k = 4) :
   clip.clust: no data provided for hclust object

Does anyone have the idea? My environment is
R version 2.10.1 (2009-12-14)
x86_64-pc-linux-gnu
[1] maptree_1.4-6

Best regards,
Keith