diff --git a/lib/ts_bm.c b/lib/ts_bm.c
index 0110e4414805440968a90a14e87c4fda8093edce..d90822c378a48af52878f41e5db18858496fddb9 100644
--- a/lib/ts_bm.c
+++ b/lib/ts_bm.c
@@ -111,15 +111,14 @@ static int subpattern(u8 *pattern, int i, int j, int g)
 	return ret;
 }
 
-static void compute_prefix_tbl(struct ts_bm *bm, const u8 *pattern,
-			       unsigned int len)
+static void compute_prefix_tbl(struct ts_bm *bm)
 {
 	int i, j, g;
 
 	for (i = 0; i < ASIZE; i++)
-		bm->bad_shift[i] = len;
-	for (i = 0; i < len - 1; i++)
-		bm->bad_shift[pattern[i]] = len - 1 - i;
+		bm->bad_shift[i] = bm->patlen;
+	for (i = 0; i < bm->patlen - 1; i++)
+		bm->bad_shift[bm->pattern[i]] = bm->patlen - 1 - i;
 
 	/* Compute the good shift array, used to match reocurrences 
 	 * of a subpattern */
@@ -150,8 +149,8 @@ static struct ts_config *bm_init(const void *pattern, unsigned int len,
 	bm = ts_config_priv(conf);
 	bm->patlen = len;
 	bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len;
-	compute_prefix_tbl(bm, pattern, len);
 	memcpy(bm->pattern, pattern, len);
+	compute_prefix_tbl(bm);
 
 	return conf;
 }