diff --git a/crypto/bytestring/bytestring_test.cc b/crypto/bytestring/bytestring_test.cc
index ca3d8350..66e9c1e0 100644
--- a/crypto/bytestring/bytestring_test.cc
+++ b/crypto/bytestring/bytestring_test.cc
@@ -596,8 +596,10 @@ static const ASN1InvalidUint64Test kASN1InvalidUint64Tests[] = {
     {"\x02\x00", 2},
     // Negative number.
     {"\x02\x01\x80", 3},
-    // Overflow
+    // Overflow.
     {"\x02\x09\x01\x00\x00\x00\x00\x00\x00\x00\x00", 11},
+    // Leading zeros.
+    {"\x02\x02\x00\x01", 4},
 };
 
 static bool TestASN1Uint64() {
diff --git a/crypto/bytestring/cbs.c b/crypto/bytestring/cbs.c
index 36d64d80..bd94ccec 100644
--- a/crypto/bytestring/cbs.c
+++ b/crypto/bytestring/cbs.c
@@ -291,7 +291,12 @@ int CBS_get_asn1_uint64(CBS *cbs, uint64_t *out) {
   }
 
   if ((data[0] & 0x80) != 0) {
-    /* negative number */
+    /* Negative number. */
+    return 0;
+  }
+
+  if (data[0] == 0 && len > 1 && (data[1] & 0x80) == 0) {
+    /* Extra leading zeros. */
     return 0;
   }