diff --git a/src/openexr.imageio/exroutput.cpp b/src/openexr.imageio/exroutput.cpp
index ccc32bc78a..dea27e18d2 100644
--- a/src/openexr.imageio/exroutput.cpp
+++ b/src/openexr.imageio/exroutput.cpp
@@ -10,6 +10,7 @@
 #include <iostream>
 #include <map>
 #include <memory>
+#include <numeric>
 
 #include <OpenImageIO/Imath.h>
 #include <OpenImageIO/platform.h>