Trouble with ifelse(), error "replacement has length zero"

I'm trying to compare different values in rows. I need to check if there's a 1, or 0, in a row, and then calculate values for a new column "error_A" according to that row. I used an ifelse() function before, like so:

df2$error_A = ifelse(df2$ns0_nns1 == 1, 1-df$A_pred, df$A_pred)

Here's a few lines of my data so it makes more sense what I'm trying to do:

essay ns0_nns1 A_pred   B_pred
 1    1        0.558     0.37
 2    0        0.293     0.654
 3    0        0.545     0.849
 4    0        0.432     0.698
 5    1        0.651     0.404

When I do this I get the error "replacement has length zero".