diff --git a/ctaocrypt/src/tfm.c b/ctaocrypt/src/tfm.c
index 25372ded9..8f2c93f3e 100644
--- a/ctaocrypt/src/tfm.c
+++ b/ctaocrypt/src/tfm.c
@@ -169,7 +169,7 @@ void s_fp_sub(fp_int *a, fp_int *b, fp_int *c)
   for (; x < a->used; x++) {
      t         = ((fp_word)a->dp[x]) - t;
      c->dp[x]  = (fp_digit)t;
-     t         = (t >> DIGIT_BIT);
+     t         = (t >> DIGIT_BIT)&1;
    }
   for (; x < oldused; x++) {
      c->dp[x] = 0;