diff --git a/include/boost/system/detail/error_code.ipp b/include/boost/system/detail/error_code.ipp
index be51fbb..6187919 100644
--- a/include/boost/system/detail/error_code.ipp
+++ b/include/boost/system/detail/error_code.ipp
@@ -11,10 +11,6 @@
 
 #include <boost/config/warning_disable.hpp>
 
-// define BOOST_SYSTEM_SOURCE so that <boost/system/config.hpp> knows
-// the library is being built (possibly exporting rather than importing code)
-#define BOOST_SYSTEM_SOURCE
-
 #include <boost/system/config.hpp>
 #include <boost/system/error_code.hpp>
 #include <boost/cerrno.hpp>
diff --git a/src/error_code.cpp b/src/error_code.cpp
index a9a8686..aa628ab 100644
--- a/src/error_code.cpp
+++ b/src/error_code.cpp
@@ -9,6 +9,10 @@
 
 //----------------------------------------------------------------------------//
 
+// define BOOST_SYSTEM_SOURCE so that <boost/system/config.hpp> knows
+// the library is being built (possibly exporting rather than importing code)
+#define BOOST_SYSTEM_SOURCE
+
 #include <boost/system/error_code.hpp>
 
 #ifndef BOOST_ERROR_CODE_HEADER_ONLY