diff --git a/src/applications/repository/engine/PhabricatorRepositoryMirrorEngine.php b/src/applications/repository/engine/PhabricatorRepositoryMirrorEngine.php
--- a/src/applications/repository/engine/PhabricatorRepositoryMirrorEngine.php
+++ b/src/applications/repository/engine/PhabricatorRepositoryMirrorEngine.php
@@ -76,6 +76,20 @@
     PhabricatorRepository $repository,
     PhabricatorRepositoryURI $mirror_uri) {
 
+    // See T5965. Test if we have any refs to mirror. If we have nothing, git
+    // will exit with an error ("No refs in common and none specified; ...")
+    // when we run "git push --mirror".
+
+    // If we don't have any refs, we just bail out. (This is arguably sort of
+    // the wrong behavior: to mirror an empty repository faithfully we should
+    // delete everything in the remote.)
+
+    list($stdout) = $repository->execxLocalCommand(
+      'for-each-ref --count 1 --');
+    if (!strlen($stdout)) {
+      return;
+    }
+
     $argv = array(
       'push --verbose --mirror -- %P',
       $mirror_uri->getURIEnvelope(),
diff --git a/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php b/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php
--- a/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php
+++ b/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php
@@ -399,6 +399,11 @@
       'ls-remote %P',
       $remote_envelope);
 
+    // Empty repositories don't have any refs.
+    if (!strlen(rtrim($stdout))) {
+      return array();
+    }
+
     $map = array();
     $lines = phutil_split_lines($stdout, false);
     foreach ($lines as $line) {