Function to generate subset of data in R - r

Question

I have created a data frame from which I'm trying to obtain a subset using some criteria as outlined below. In simple terms, I'd like to write a function to select half of all male ids below the age of 4. I need this function subsequently to do other things for me hence the need for writing a function.
I've tried to write a simple code as given below but it seems an error is coming back-I would really appreciate some help with this. I think this error is coming from the filter() task but I have no idea why.
library(dplyr)
set.seed(2016)
longData <- data.frame(patient=rep(paste(letters[1:20], sep = "_", 1:20), each=5),
age=rep(runif(20, 1, 10), each = 5),var=runif(100, 50, 1000),
time=rep(1:5, 20), gender = factor(rep(c("male", "female"), c(60, 40))))
ptNo <- function(data, id = "patient")
{
ptNumber <- data %>% filter(!duplicated(data[id])) %>% nrow()
return(ptNumber)
}
randSubsetFun <- function(df, gender, seedNumb){
data <- filter(df, gender, age < 4)
set.seed(seedNumb)
n <- round(ptNo(data)/2)
fd <- sample_n(data, n)
return(fd)
}
randSubsetFun(df = longData, gender = "male", seedNumb = 1)
The error reads the following:
Warning message:
In Ops.factor(c(2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, :
‘&’ not meaningful for factors
The annoying thing is when I tried running these codes individually outside of the function, it worked fine. It just doesn't work within the function.
Any help would be warmly appreciated. Any suggestions as to how I can improve this code would be welcome too. I guess the problem really is how do I call a factor from the filter function via this function (If that makes sense)?