diff --git a/lib/mu-xapian-db.hh b/lib/mu-xapian-db.hh
index 5a691c47..5daf363d 100644
--- a/lib/mu-xapian-db.hh
+++ b/lib/mu-xapian-db.hh
@@ -25,6 +25,7 @@
 #include <string>
 #include <mutex>
 #include <functional>
+#include <unordered_map>
 
 #include <glib.h>