diff --git a/lib/crypto/index.js b/lib/crypto/index.js
index 68f240acb..503e88a11 100644
--- a/lib/crypto/index.js
+++ b/lib/crypto/index.js
@@ -633,6 +633,11 @@ Crypto.prototype.getEventSenderDeviceInfo = function(event) {
         event.getSender(), algorithm, sender_key
     );
 
+    if (device === null) {
+        // we haven't downloaded the details of this device yet.
+        return null;
+    }
+
     // so far so good, but now we need to check that the sender of this event
     // hadn't advertised someone else's Curve25519 key as their own. We do that
     // by checking the Ed25519 claimed by the event (or, in the case of megolm,