Error message running the example from the reshape

My code, which was running without problems before, crashed when calling the dcast formula. After playing around, I found out that even the minimal example from the help page of dcast does not work for me anymore. More precisely:

#Air quality example
names(airquality) <- tolower(names(airquality))
aqm <- melt(airquality, id=c("month", "day"), na.rm=TRUE)

acast(aqm, day ~ month ~ variable)
acast(aqm, month ~ variable, mean)

The last line produces the following error:

Error in vaggregate(.value = value, .group = overall, .fun = fun.aggregate,  : 
  could not find function ".fun"

Here is my sessionInfo():

R version 2.13.1 (2011-07-08)
Platform: x86_64-pc-mingw32/x64 (64-bit)

locale:
[1] C

attached base packages:
[1] grid      stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] ggplot2_0.8.9   proto_0.3-9.2   reshape2_1.1    xtable_1.5-6    reshape_0.8.4   plyr_1.5.2      lubridate_0.2.5

loaded via a namespace (and not attached):
[1] stringr_0.5  tools_2.13.1

I don't come up with a satisfying answer of what is going wrong here, so I would appreciate some help. Also, I found the following thread here on stackoverflow: Similar problem This problem seems to result from a custom function. I, however, use the standard mean function and a standard example from the help page.

UPDATE: I just did some internet research and did not find any information regarding an update of the reshape2 package. This was the best guess I had regarding the problem.

UPDATE2: The problem occurred because I most probably reassigned the mean function while I was playing around with a statisctic example during an R session. Restarting R solved the problem. Now, everything works as expected again.

For completeness:

PaulHurleyuk's comment:

Have you tried restarting R and trying the example in a fresh session ? Or do rm(list=ls()) to remove everything from the current session. In the past I have managed to break things by assigning something to something that shouldn't be assigned to.

Christoph_J's response:

Thanks ... that was exactly the problem...

The problem occurred because I most probably reassigned the mean function while I was playing around with a statisctic example during an R session. Restarting R solved the problem. Now, everything works as expected again.