diff --git a/include/nlohmann/detail/output/fancy_serializer.hpp b/include/nlohmann/detail/output/fancy_serializer.hpp
index ee6a71d28..684a0852f 100644
--- a/include/nlohmann/detail/output/fancy_serializer.hpp
+++ b/include/nlohmann/detail/output/fancy_serializer.hpp
@@ -13,6 +13,7 @@
 #include <string> // string
 #include <type_traits> // is_same
 #include <map>
+#include <sstream>
 
 #include <nlohmann/detail/exceptions.hpp>
 #include <nlohmann/detail/conversions/to_chars.hpp>
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index c1ede78ab..aef95084a 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -10076,6 +10076,7 @@ class serializer
 #include <string> // string
 #include <type_traits> // is_same
 #include <map>
+#include <sstream>
 
 // #include <nlohmann/detail/exceptions.hpp>