about data manipulation

Hi R users,

I'm trying to manipulate dataset, but met some difficulties.

df
year   month   flow
2006     3        3.5
2006     4        3.8
2006     5        21
2006     6        32
2007     3        4.1
2007     4        4.4
...

I want to calculate total flow for each year, and use the code below:
aggregate(flow~year, data=df, sum)
But it gave the error message:
Error in get(as.character(FUN), mode = "function", envir = envir) :
  object 'FUN' of mode 'function' was not found

What is the problem and how to solve it? Thanks for your help.