diff --git a/luna/include/luna/Result.h b/luna/include/luna/Result.h
index 38143c7e..ed2935e3 100644
--- a/luna/include/luna/Result.h
+++ b/luna/include/luna/Result.h
@@ -1,4 +1,5 @@
 #pragma once
+#define _LUNA_SYSTEM_ERROR_EXTENSIONS
 #include <luna/Check.h>
 #include <luna/Move.h>
 #include <luna/Option.h>
diff --git a/luna/include/luna/SystemError.h b/luna/include/luna/SystemError.h
index 4c93724b..a6aba0c3 100644
--- a/luna/include/luna/SystemError.h
+++ b/luna/include/luna/SystemError.h
@@ -54,7 +54,10 @@
 #define ETIMEDOUT 110   // Connection timed out
 #define EALREADY 114    // Operation already in progress
 
+#if defined(IN_MOON) || defined(USE_FREESTANDING) || defined(_LUNA_SYSTEM_ERROR_EXTENSIONS)
 // This one is Luna-specific.
 #define EFIXME 342 // Functionality not yet implemented
 
-const char* error_string(int error);
\ No newline at end of file
+const char* error_string(int error);
+
+#endif
\ No newline at end of file
diff --git a/luna/src/TarStream.cpp b/luna/src/TarStream.cpp
index 70093ac3..ecc85e80 100644
--- a/luna/src/TarStream.cpp
+++ b/luna/src/TarStream.cpp
@@ -1,3 +1,4 @@
+#define _LUNA_SYSTEM_ERROR_EXTENSIONS
 #include <luna/Alignment.h>
 #include <luna/CString.h>
 #include <luna/NumberParsing.h>