diff --git a/Assistant/Ssh.hs b/Assistant/Ssh.hs
index 876f854667..59ed34497a 100644
--- a/Assistant/Ssh.hs
+++ b/Assistant/Ssh.hs
@@ -223,6 +223,10 @@ knownHost :: Text -> IO Bool
 knownHost hostname = do
 	sshdir <- sshDir
 	ifM (doesFileExist $ sshdir </> "known_hosts")
-		( not . null <$> readProcess "ssh-keygen" ["-F", T.unpack hostname]
+		( not . null <$> checkhost
 		, return False
 		)
+	where
+		{- ssh-keygen -F can crash on some old known_hosts file -}
+		checkhost = catchDefaultIO "" $
+			readProcess "ssh-keygen" ["-F", T.unpack hostname]
diff --git a/debian/changelog b/debian/changelog
index 976b70903d..f8efeba5aa 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -16,6 +16,8 @@ git-annex (3.20120925) UNRELEASED; urgency=low
     in the git repository, as that would allow anyone with access to
     the repository access to the S3 account. Instead, they're stored
     in a 600 mode file in the local git repo.
+  * webapp: Avoid crashing when ssh-keygen -F chokes on an invalid known_hosts
+    file.
 
  -- Joey Hess <joeyh@debian.org>  Mon, 24 Sep 2012 19:58:07 -0400