"The process is forked ..." Error using tm package in R

Question

I set tm packageto Rto perform text analysis. After creating the enclosure, I wanted to use a function tm_map()that produces the following error message:

The process has forked and you cannot use this CoreFoundation functionality safely. You MUST exec().Break on 

Does anyone know why this message appears?

Here is more code to explain:

> require(tm)
Lade nötiges Paket: tm
> a <-Corpus(VectorSource(chi2014_df$text))
> a
A corpus with 70 text documents
> a <-tm_map(a,tolower)
The process has forked and you cannot use this CoreFoundation functionality safely. You MUST exec().
Break on 

I am using R 3.0.1 GUI 1.61 Snow Leopard build (6492) on OSX 10.7.5

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