diff --git a/toml/region.hpp b/toml/region.hpp
index 2fb1eaa..923da9b 100644
--- a/toml/region.hpp
+++ b/toml/region.hpp
@@ -9,6 +9,7 @@
 #include <initializer_list>
 #include <iterator>
 #include <iomanip>
+#include <cassert>
 
 namespace toml
 {