diff --git a/src/libecoprimer/pairs.c b/src/libecoprimer/pairs.c
index a11030d..88fc9f5 100644
--- a/src/libecoprimer/pairs.c
+++ b/src/libecoprimer/pairs.c
@@ -188,7 +188,7 @@ static void buildPrimerPairsForOneSeq(uint32_t seqid,
 	bool_t             strand;
 	//char 			   prmr[50];
 	//float			   mtemp;
-	word_t 			   w1, w1a, omask = (0x1L << options->strict_three_prime) -1;;
+	word_t 			   w1, w1a, omask = (0x1L << (options->strict_three_prime*2)) -1;
 	word_t 			   w2, w2a, wtmp;
 	uint32_t   		   bp1,bp2;