diff --git a/lib/utils/mu-sexp-parser.hh b/lib/utils/mu-sexp-parser.hh
index ea7b90b4..104be595 100644
--- a/lib/utils/mu-sexp-parser.hh
+++ b/lib/utils/mu-sexp-parser.hh
@@ -22,6 +22,8 @@
 
 #include <string>
 #include <vector>
+#include <ostream>
+
 
 #include "utils/mu-error.hh"