[R] help! Error in using Boosting...

[Michael]

Here is my code:
mygbm<-gbm.fit(y=mytraindata[, 1], x=mytraindata[, -1],
interaction.depth=4, shrinkage=0.001, n.trees=20000, bag.fraction=1,
distribution="bernoulli")

Here is the error:
Error in gbm.fit(y = mytraindata[, 1], x = mytraindata[, -1],
interaction.depth = 4,  :
  The dataset size is too small or subsampling rate is too large:
cRows*train.fraction*bag.fraction <= n.minobsinnode

What might be the problem?

Thanks a lot!