diff --git a/libs/pbd/xml++.cc b/libs/pbd/xml++.cc
index c42c1d39a2..797f09e78b 100644
--- a/libs/pbd/xml++.cc
+++ b/libs/pbd/xml++.cc
@@ -5,6 +5,7 @@
  * Modified for Ardour and released under the same terms.
  */
 
+#include <cassert>
 #include <string.h>
 #include <iostream>