diff --git a/crypto/shash.c b/crypto/shash.c
index 359754591653..b2cd109d9171 100644
--- a/crypto/shash.c
+++ b/crypto/shash.c
@@ -274,12 +274,14 @@ static int shash_async_finup(struct ahash_request *req)
 
 int shash_ahash_digest(struct ahash_request *req, struct shash_desc *desc)
 {
-	struct scatterlist *sg = req->src;
-	unsigned int offset = sg->offset;
 	unsigned int nbytes = req->nbytes;
+	struct scatterlist *sg;
+	unsigned int offset;
 	int err;
 
-	if (nbytes < min(sg->length, ((unsigned int)(PAGE_SIZE)) - offset)) {
+	if (nbytes &&
+	    (sg = req->src, offset = sg->offset,
+	     nbytes < min(sg->length, ((unsigned int)(PAGE_SIZE)) - offset))) {
 		void *data;
 
 		data = kmap_atomic(sg_page(sg));