diff --git a/src/cryptography/x509.py b/src/cryptography/x509.py
index 6c9cd56..f54eccf 100644
--- a/src/cryptography/x509.py
+++ b/src/cryptography/x509.py
@@ -691,6 +691,7 @@
         spki, remaining = decoder.decode(
             serialized, asn1Spec=_SubjectPublicKeyInfo()
         )
+        assert not remaining
         # the univ.BitString object is a tuple of bits. We need bytes and
         # pyasn1 really doesn't want to give them to us. To get it we'll
         # build an integer, hex it, then decode the hex.