diff --git a/src/fido/cbor_large_blobs.c b/src/fido/cbor_large_blobs.c
index 3b79296..ed2ec46 100644
--- a/src/fido/cbor_large_blobs.c
+++ b/src/fido/cbor_large_blobs.c
@@ -130,7 +130,7 @@ int cbor_large_blobs(const uint8_t *data, size_t len) {
         expectedNextOffset += set.len;
         if (expectedNextOffset == expectedLength) {
             uint8_t sha[32];
-            mbedtls_sha256(temp_lba, expectedLength, sha, 0);
+            mbedtls_sha256(temp_lba, expectedLength-16, sha, 0);
             if (expectedLength > 17 && memcmp(sha, temp_lba+expectedLength-16, 16) != 0)
                 CBOR_ERROR(CTAP2_ERR_INTEGRITY_FAILURE);
             flash_write_data_to_file(ef_largeblob, temp_lba, expectedLength);