diff --git a/core/src/histogram.cc b/core/src/histogram.cc
index 4a40d535..80235298 100644
--- a/core/src/histogram.cc
+++ b/core/src/histogram.cc
@@ -4,6 +4,7 @@
 #include <cassert>
 #include <iterator>
 #include <numeric>
+#include <ostream>
 
 namespace prometheus {