diff --git a/slixmpp_omemo/__init__.py b/slixmpp_omemo/__init__.py
index 841a46a..e736017 100644
--- a/slixmpp_omemo/__init__.py
+++ b/slixmpp_omemo/__init__.py
@@ -16,6 +16,7 @@ from typing import Any, Dict, List, Optional, Set, Tuple, Union
 # Not available in Python 3.7, and slixmpp already imports the right things
 # for me
 from slixmpp.types import TypedDict
+from functools import reduce
 
 import os
 import json
@@ -533,10 +534,12 @@ class XEP_0384(BasePlugin):
         for did in devices:
             session = self._omemo._SessionManager__loadSession(bare, did)
             if session is None:
-                break
+                continue
             skr = session._DoubleRatchet__skr
-            lengths['sending'].append((did, skr.sending_chain_length))
-            lengths['receiving'].append((did, skr.receiving_chain_length))
+            sending = skr.sending_chain_length or -1
+            receiving = skr.receiving_chain_length or -1
+            lengths['sending'].append((did, sending))
+            lengths['receiving'].append((did, receiving))
 
         return lengths
 
@@ -545,11 +548,16 @@ class XEP_0384(BasePlugin):
             Returns whether we should send a heartbeat message for JID.
             See notes about heartbeat in
             https://xmpp.org/extensions/xep-0384.html#rules.
+
+            This method will return True when a session among all of the
+            sessions for this JID is not yet confirmed, or if one of the
+            sessions hasn't been answered in a while.
         """
 
         receiving_chain_lengths = self._chain_lengths(jid).get('receiving', [])
         lengths = map(lambda d_l: d_l[1], receiving_chain_lengths)
-        return max(lengths, default=0) > self.heartbeat_after
+        min_length = reduce(lambda x, d_l: min(x, d_l[1]), receiving_chain_lengths, 0) == -1
+        return min_length or max(lengths, default=0) > self.heartbeat_after
 
     async def make_heartbeat(self, jid: JID) -> Message:
         """