diff --git a/include/htool/hmatrix/hmatrix_output.hpp b/include/htool/hmatrix/hmatrix_output.hpp
index 9924db38..7b9f77aa 100644
--- a/include/htool/hmatrix/hmatrix_output.hpp
+++ b/include/htool/hmatrix/hmatrix_output.hpp
@@ -3,6 +3,7 @@
 
 #include <array>
 #include <fstream>
+#include <iostream>
 #include <map>
 #include <vector>
 #if defined(_OPENMP)