commit 8d57be27ddeda68cbc0f97ee857871aadb5c5170
Author: Jared Hobbs <jared@pyhacker.com>
Date:   Wed Mar 20 10:08:12 2019 -0600

    handle exception that can occur if no regex match

diff --git a/paramiko/pkey.py b/paramiko/pkey.py
index f2447e15..0464795a 100644
--- a/paramiko/pkey.py
+++ b/paramiko/pkey.py
@@ -307,8 +307,8 @@ class PKey(object):
             start += 1
             m = self.BEGIN_TAG.match(lines[start])
         start += 1
-        keytype = m.group(1)
-        if start >= len(lines):
+        keytype = m.group(1) if m else None
+        if start >= len(lines) or keytype is None:
             raise SSHException("not a valid " + tag + " private key file")
 
         # find the END tag