diff --git a/rabin/rabin_dedup.c b/rabin/rabin_dedup.c
index aed8989..ad32df8 100755
--- a/rabin/rabin_dedup.c
+++ b/rabin/rabin_dedup.c
@@ -180,7 +180,7 @@ create_dedupe_context(uint64_t chunksize, uint64_t real_chunksize, int rab_blk_s
 	ctx->fixed_flag = fixed_flag;
 	ctx->rabin_break_patt = 0;
 	ctx->rabin_poly_avg_block_size = 1 << (rab_blk_sz + RAB_BLK_MIN_BITS);
-	ctx->rabin_avg_block_mask = ((ctx->rabin_poly_avg_block_size - 1) >> 2);
+	ctx->rabin_avg_block_mask = RAB_BLK_MASK;
 	ctx->rabin_poly_min_block_size = dedupe_min_blksz(rab_blk_sz);
 	ctx->delta_flag = 0;
 
diff --git a/rabin/rabin_dedup.h b/rabin/rabin_dedup.h
index 37b1a55..7aab87f 100644
--- a/rabin/rabin_dedup.h
+++ b/rabin/rabin_dedup.h
@@ -73,6 +73,7 @@
 #define	RAB_POLYNOMIAL_MIN_WIN_SIZE 8
 #define	RAB_POLYNOMIAL_MAX_WIN_SIZE 64
 #define	RAB_POLYNOMIAL_MAX_BLOCK_SIZE (128 * 1024)
+#define	RAB_BLK_MASK (((1 << RAB_BLK_MIN_BITS) - 1) >> 1)
 
 // Minimum practical chunk size when doing dedup
 #define	RAB_MIN_CHUNK_SIZE (1048576L)