diff --git a/app-indexeddb-mirror/app-indexeddb-mirror-client.html b/app-indexeddb-mirror/app-indexeddb-mirror-client.html
index 5fec36b..5f6d8e7 100644
--- a/app-indexeddb-mirror/app-indexeddb-mirror-client.html
+++ b/app-indexeddb-mirror/app-indexeddb-mirror-client.html
@@ -57,6 +57,12 @@
        * the session has been validated.
        */
       validateSession: function(session) {
+        if (session === undefined) {
+          // Ignore session `undefined` conventionally, as it means the session
+          // (such as the user ID) has not been initialized yet.
+          return;
+        }
+
         return this.post({
           type: 'app-mirror-validate-session',
           session: session
diff --git a/test/app-indexeddb-mirror/app-indexeddb-mirror.html b/test/app-indexeddb-mirror/app-indexeddb-mirror.html
index 3b9d8d0..58a0ecc 100644
--- a/test/app-indexeddb-mirror/app-indexeddb-mirror.html
+++ b/test/app-indexeddb-mirror/app-indexeddb-mirror.html
@@ -35,7 +35,15 @@
     <template>
       <app-indexeddb-mirror
           session="test"
-          key="basic-mirror" log>
+          key="basic-mirror">
+      </app-indexeddb-mirror>
+    </template>
+  </test-fixture>
+
+  <test-fixture id="NoSessionMirror">
+    <template>
+      <app-indexeddb-mirror
+          key="basic-mirror">
       </app-indexeddb-mirror>
     </template>
   </test-fixture>
@@ -113,6 +121,32 @@
           });
         });
 
+        suite('when session is set lazily', function() {
+          var secondMirror;
+
+          setup(function() {
+            mirror.data = {
+              foo: 'bar'
+            };
+            secondMirror = fixture('NoSessionMirror');
+            return Promise.all([
+              mirror.transactionsComplete,
+              secondMirror.transactionsComplete
+            ]);
+          });
+
+          test('the data is preserved', function() {
+            if (!mirror.client.supportsMirroring) {
+              return;
+            }
+
+            secondMirror.session = 'test';
+            return secondMirror.transactionsComplete.then(function() {
+              return expectPersistedValue('basic-mirror', mirror.data);
+            });
+          });
+        });
+
         suite('when offline', function() {
           test('boots up with persisted value', function() {
             if (!mirror.client.supportsMirroring) {