R programming error missing value where TRUE FALSE needed

Question

My code
tfr<-function(x) {return (x[1]^2 + x[2]^2 + x[3]^2 <=1)}
monte_carlo<-function (test_fn, n,  x0=-1, y0=-1, z0=-1, x1=1, y1=1, z1=1)
{
  V_B<- ((x1-x0)*(y1-y0)*(z1-z0))
  k<-0
  for (I in 1:n)
  {
     x<- runif(2, min=c(x0,y0,z0), max=c(x1,y1,z1))  
    if (test_fn(x))
     { k<- k+1}
  }
  return (V_B*k/n)
}

at startup
> monte_carlo(tfr,100000)
gives out
Error in if (test_fn(x)) { : missing value where TRUE/FALSE needed