diff --git a/block-sha1/sha1.c b/block-sha1/sha1.c
index 22b125cf8c..8681031402 100644
--- a/block-sha1/sha1.c
+++ b/block-sha1/sha1.c
@@ -203,7 +203,7 @@ void blk_SHA1_Init(blk_SHA_CTX *ctx)
 	ctx->H[4] = 0xc3d2e1f0;
 }
 
-void blk_SHA1_Update(blk_SHA_CTX *ctx, const void *data, unsigned long len)
+void blk_SHA1_Update(blk_SHA_CTX *ctx, const void *data, size_t len)
 {
 	unsigned int lenW = ctx->size & 63;
 
diff --git a/block-sha1/sha1.h b/block-sha1/sha1.h
index 4df6747752..9fb0441b98 100644
--- a/block-sha1/sha1.h
+++ b/block-sha1/sha1.h
@@ -13,7 +13,7 @@ typedef struct {
 } blk_SHA_CTX;
 
 void blk_SHA1_Init(blk_SHA_CTX *ctx);
-void blk_SHA1_Update(blk_SHA_CTX *ctx, const void *dataIn, unsigned long len);
+void blk_SHA1_Update(blk_SHA_CTX *ctx, const void *dataIn, size_t len);
 void blk_SHA1_Final(unsigned char hashout[20], blk_SHA_CTX *ctx);
 
 #define platform_SHA_CTX	blk_SHA_CTX