diff --git a/src/engine/SX-book.c b/src/engine/SX-book.c
index 33e3822697..78fc025493 100644
--- a/src/engine/SX-book.c
+++ b/src/engine/SX-book.c
@@ -45,6 +45,7 @@
 #include "SX-book.h"
 #include "SX-book-p.h"
 #include "gnc-event.h"
+#include <qofinstance-p.h>
 
 #undef G_LOG_DOMAIN
 #define G_LOG_DOMAIN "gnc.engine.sx"