diff --git a/hardware/arduino/cores/arduino/WString.h b/hardware/arduino/cores/arduino/WString.h
index d76d2a33d..947325e5f 100644
--- a/hardware/arduino/cores/arduino/WString.h
+++ b/hardware/arduino/cores/arduino/WString.h
@@ -35,7 +35,7 @@
 //     -std=c++0x
 
 class __FlashStringHelper;
-#define F(string_literal) (reinterpret_cast<__FlashStringHelper *>(PSTR(string_literal)))
+#define F(string_literal) (reinterpret_cast<const __FlashStringHelper *>(PSTR(string_literal)))
 
 // An inherited class for holding the result of a concatenation.  These
 // result objects are assumed to be writable by subsequent concatenations.