diff --git a/docs/development/test-vectors.rst b/docs/development/test-vectors.rst
index 0d1eea1..b189bc9 100644
--- a/docs/development/test-vectors.rst
+++ b/docs/development/test-vectors.rst
@@ -193,6 +193,8 @@
   DNS name entries of the SAN extension.
 * ``badasn1time.pem`` - A certificate containing an incorrectly specified
   UTCTime in its validity->not_after.
+* ``letsencryptx3.pem`` - A subordinate certificate used by Let's Encrypt to
+  issue end entity certificates.
 
 Custom X.509 Vectors
 ~~~~~~~~~~~~~~~~~~~~