diff --git a/N2kDef.h b/N2kDef.h
index 8f79fc53..15b909de 100644
--- a/N2kDef.h
+++ b/N2kDef.h
@@ -58,7 +58,9 @@ extern uint32_t millis();
 class __FlashStringHelper;
 #define F(str) (reinterpret_cast<const __FlashStringHelper*>(PSTR(str)))
 #else
+#ifndef F
 #define F(str) str
 #endif
+#endif
 
 #endif