--- a/src/dsl_cpe_os.h
+++ b/src/dsl_cpe_os.h
@@ -31,15 +31,15 @@
 #ifndef _lint
 #ifdef LINUX
 #include "dsl_cpe_linux.h"
-#elif WIN32
+#elif defined(WIN32)
 #include "dsl_cpe_win32.h"
-#elif VXWORKS
+#elif defined(VXWORKS)
 #include "dsl_cpe_vxworks.h"
-#elif ECOS
+#elif defined(ECOS)
 #include "dsl_cpe_ecos.h"
-#elif RTEMS
+#elif defined(RTEMS)
 #include "dsl_cpe_rtems.h"
-#elif GENERIC_OS
+#elif defined(GENERIC_OS)
 #include "dsl_cpe_generic_os.h"
 #else
 #error please define your OS for the CPE Control adaptation
