Why replacement has length zero? And How can I fix it?

Hi, all,

I am working on a project to run a simulation. I am now working on the
main body of the simulation body, I generate the data and all work well
before I identify a variable "IAP", there is always an error message as
"replacement has length zero", and I do not know how to fix it. Can anyone
help?

Here is the code:

library (mvtnorm)

options(digits=2)

# Set variable name #
namelist1 <- c("Age", "Gender", "SES", "ISS", "IQTe", "PreTe")

# Generate Data #
Sample.Size <- 10 # Variable Sample Size #
CorMat <- matrix(c(1.0 ,0.0 ,0.0 , 0.5, 0.2, 0.5,
					0.0, 1.0 ,0.0, 0.0, 0.0, 0.0,
					0.0 ,0.0 ,1.0, 0.5, 0.4, 0.5,
					0.5, 0.0, 0.5, 1.0, 0.4, 0.7,
					0.2, 0.0, 0.4, 0.4, 1.0, 0.9,
					0.5, 0.0, 0.5, 0.7, 0.9, 1.0),ncol=6) # Correlation Matrix #

DataSet <- rmvnorm(Sample.Size, mean=c(0,0,0,0,0,0), sigma=CorMat) # Draw
Sample.Size covariate set all mean 0 #
DataSet <- as.data.frame(DataSet)
names(DataSet) <- namelist1
DataSet

# For Treatment IAP #
IAP <- matrix(0, nrow=Sample.Size)
for (i in 1:Sample.Size){
	IAP[i] <- if (DataSet$SES>0)
	{
		ifelse(runif[1]>0.75, 1, 0) # High SES, higher chance to be in Treatment
#
	}
	IAP[i] <- if (DataSet$SES<=0)
	{
		ifelse(runif[1]<=0.25, 1, 0) # Low SES, lower chance to be in Treatment
#
	}
} # End loop #
IAP

Error in IAP[i] <- if (DataSet$SES > 0) { : replacement has length zero
In addition: Warning message:
In if (DataSet$SES > 0) { :
  the condition has length > 1 and only the first element will be used

Thanks!