Error in if() missing value where TRUE/FALSE needed, but there's actually a boolean argument inside

I have this error in R. I'm working with a database, where my NA values are called "# VALOR N/A", so I did a simple function to see how many of this values I have.

    estavencido <- function(a){
  count = 0
  for(i in 2:367){
    if(a[i]== "# VALOR N/D"){
      count = count+1
    }
  }
 return(count)
}

But when I use the function, I get the error, and I saw that data[i]=="# VALOR N/D" returns a TRUE/FALSE value, so I don't know why this is happening.