diff --git a/libs/pbd/tlsf.cc b/libs/pbd/tlsf.cc
index 13709ecbfb..5a83159731 100644
--- a/libs/pbd/tlsf.cc
+++ b/libs/pbd/tlsf.cc
@@ -56,6 +56,7 @@
 #endif
 
 #include <assert.h>
+#include <stdlib.h>
 #include <string.h>
 
 #ifndef TLSF_STATISTIC