diff --git a/include/htool/hmatrix/hmatrix_output.hpp b/include/htool/hmatrix/hmatrix_output.hpp
index a1fac293..c1c528a2 100644
--- a/include/htool/hmatrix/hmatrix_output.hpp
+++ b/include/htool/hmatrix/hmatrix_output.hpp
@@ -1,6 +1,7 @@
 #ifndef HTOOL_HMATRIX_OUTPUT_HPP
 #define HTOOL_HMATRIX_OUTPUT_HPP
 
+#include <algorithm>
 #include <array>
 #include <fstream>
 #include <iomanip>