diff --git a/src/common/extern_data.cpp b/src/common/extern_data.cpp
index fd79e9848..47a49b142 100644
--- a/src/common/extern_data.cpp
+++ b/src/common/extern_data.cpp
@@ -16,7 +16,9 @@
 #include "os.h"
 #include "extern_data.h"
 
-#define NULL 0
+#ifndef NULL
+# define NULL 0
+#endif
 
 #ifdef SYS_UNIX
 const char *sub_charsets[] = {