diff --git a/mlpp/outlier_finder/outlier_finder_old.h b/mlpp/outlier_finder/outlier_finder_old.h
index 3db4349..97619d5 100644
--- a/mlpp/outlier_finder/outlier_finder_old.h
+++ b/mlpp/outlier_finder/outlier_finder_old.h
@@ -9,6 +9,7 @@
 //
 
 #include "core/math/math_defs.h"
+#include "core/int_types.h"
 
 #include <vector>