diff --git a/agents/unifiedmailboxagent/unifiedmailboxmanager.cpp b/agents/unifiedmailboxagent/unifiedmailboxmanager.cpp
--- a/agents/unifiedmailboxagent/unifiedmailboxmanager.cpp
+++ b/agents/unifiedmailboxagent/unifiedmailboxmanager.cpp
@@ -388,8 +388,12 @@
             if (!isUnifiedMailbox(col) || col.parentCollection() == Akonadi::Collection::root()) {
                 continue;
             }
-
-            mMailboxes.at(col.name())->setCollectionId(col.id());
+            const auto it = mMailboxes.find(col.name());
+            if (it == mMailboxes.end()) {
+                qCWarning(UNIFIEDMAILBOXAGENT_LOG) << "Failed to find an unified mailbox for source collection" << col.id();
+            } else {
+                it->second->setCollectionId(col.id());
+            }
         }
     });
     if (finishedCb) {