Error using the "prob" package in an R function

Error using the "prob" package in an R function

I'm attempting to write a function that uses the prob package to compute
conditional probabilities. When using the function I continue to encounter
the same error, which states an object within the function cannot be
found.
Below is a reproducible example in which I compute a conditional
probability without the function and then attempt to use the function to
produce the same result. I'm not sure if the error is due to limitations
with the prob package or an error on my part.
# Load prob package
library(prob)
# Set seed for reproducibility
set.seed(30)
# Sample data frame
sampledata <- data.frame(
X <- sample(1:10),
Y <- sample(c(-1, 0, 1), 10, replace=TRUE))
# Set probability space
S <- probspace(sampledata)
# Subset Y between -1 and 0
A <- subset(S, Y>=-1 & Y<=0)
# Subset X greater than 6
B <- subset(S, X>6)
# Compute conditional probability
P <- prob(A, given=B)
The above code produces the following probability:
> P
[1] 0.25
Attempting to write a function to calculate the same probability:
# Create function with data frame, variables, and conditional inputs
prob.function <- function(df, variable1, variable2, state1, state2, cond1){
s <- probspace(df)
a <- subset(s, variable1>=state1 & variable1<=state2)
b <- subset(s, variable2>cond1)
p <- prob(a, given=b)
return(p)
}
# Demonstrate the function
test <- prob.function(sampledata, Y, X, -1, 0, 6)
This function gives the following error:
Error in eval(expr, envir, enclos) : object 'b' not found
Any help you can provide would be great.
Thanks!