diff --git a/libraries/AP_Common/AP_Common.h b/libraries/AP_Common/AP_Common.h
index d65245f17c..b93c862eb1 100644
--- a/libraries/AP_Common/AP_Common.h
+++ b/libraries/AP_Common/AP_Common.h
@@ -84,6 +84,11 @@ typedef struct {
                 (prog_char_t *)&__c[0];}))
 #endif
 
+// a varient of PSTR() for progmem strings passed to %S in printf()
+// this gets the gcc __format__ checking right
+#define FPSTR(s) (wchar_t *)(s)
+
+
 static inline int strcasecmp_P(const char *str1, const prog_char_t *pstr)
 {
 	return strcasecmp_P(str1, (const prog_char *)pstr);
diff --git a/libraries/AP_Common/Arduino.mk b/libraries/AP_Common/Arduino.mk
index 084580beba..6f5e956dfa 100644
--- a/libraries/AP_Common/Arduino.mk
+++ b/libraries/AP_Common/Arduino.mk
@@ -159,7 +159,7 @@ endif
 # Tool options
 #
 DEFINES			=	-DF_CPU=$(F_CPU) -DARDUINO=$(ARDUINO_VERS)
-OPTFLAGS		=	-Os
+OPTFLAGS		=	-Os -Wformat
 DEPFLAGS		=	-MD -MT $@
 
 # XXX warning options TBD
diff --git a/libraries/AP_Common/menu.cpp b/libraries/AP_Common/menu.cpp
index 499e3ff968..a4ea664a4f 100644
--- a/libraries/AP_Common/menu.cpp
+++ b/libraries/AP_Common/menu.cpp
@@ -44,7 +44,7 @@ Menu::run(void)
 
 		// loop reading characters from the input
 		len = 0;
-		Serial.printf("%S] ", _prompt);
+		Serial.printf("%S] ", FPSTR(_prompt));
 		for (;;) {
 			c = Serial.read();
 			if (-1 == c)
@@ -127,7 +127,7 @@ Menu::_help(void)
 	
 	Serial.println("Commands:");
 	for (i = 0; i < _entries; i++)
-		Serial.printf("  %S\n", _commands[i].command);
+		Serial.printf("  %S\n", FPSTR(_commands[i].command));
 }
 
 // run the n'th command in the menu