diff --git a/tests/test_format_error.cpp b/tests/test_format_error.cpp
index 7e6c290..df558da 100644
--- a/tests/test_format_error.cpp
+++ b/tests/test_format_error.cpp
@@ -6,6 +6,7 @@
 #include <boost/test/included/unit_test.hpp>
 #endif
 #include <toml.hpp>
+#include <iostream>
 
 // to check it successfully compiles. it does not check the formatted string.