diff --git a/lib/database-private.h b/lib/database-private.h
index 5bb6e86c..41918d76 100644
--- a/lib/database-private.h
+++ b/lib/database-private.h
@@ -21,6 +21,15 @@
 #ifndef NOTMUCH_DATABASE_PRIVATE_H
 #define NOTMUCH_DATABASE_PRIVATE_H
 
+/* According to WG14/N1124, a C++ implementation won't provide us a
+ * macro like PRIx64 (which gives a printf format string for
+ * formatting a uint64_t as hexadecimal) unless we define
+ * __STDC_FORMAT_MACROS before including inttypes.h. That's annoying,
+ * but there it is.
+ */
+#define __STDC_FORMAT_MACROS
+#include <inttypes.h>
+
 #include "notmuch-private.h"
 
 #include <xapian.h>
diff --git a/lib/database.cc b/lib/database.cc
index 86413215..2b5b64df 100644
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -1306,7 +1306,7 @@ _notmuch_database_generate_thread_id (notmuch_database_t *notmuch)
 
     notmuch->last_thread_id++;
 
-    sprintf (thread_id, "%016llx", notmuch->last_thread_id);
+    sprintf (thread_id, "%016" PRIx64, notmuch->last_thread_id);
 
     db->set_metadata ("last_thread_id", thread_id);