diff --git a/Makefile-ARM b/Makefile-ARM
index ddb2932..6af50ac 100644
--- a/Makefile-ARM
+++ b/Makefile-ARM
@@ -97,7 +97,7 @@ TARGET = $(PROGRAM).hex
 #SOURCES = $(wildcard *.c)
 # Until the generic ARM port is completed, we'd have to wrap all sources
 # in #ifdef __AVR__. To avoid this, build only a selection for now:
-SOURCES  = mendel.c cpu.c serial.c sermsg.c
+SOURCES  = mendel.c cpu.c serial.c sermsg.c sersendf.c
 SOURCES += mbed-pinmap.c
 ifeq ($(MCU), lpc1114)
   SOURCES += mbed-system_LPC11xx.c
diff --git a/mendel.c b/mendel.c
index fd35061..a856a79 100644
--- a/mendel.c
+++ b/mendel.c
@@ -40,7 +40,6 @@
 #include	"temp.h"
 #include	"watchdog.h"
 #include	"debug.h"
-#include	"sersendf.h"
 #include	"heater.h"
 #include	"analog.h"
 #include	"pinio.h"
diff --git a/sersendf.c b/sersendf.c
index 46af1d9..0149d6e 100644
--- a/sersendf.c
+++ b/sersendf.c
@@ -94,10 +94,15 @@
 	\code sersendf_P(PSTR("X:%ld Y:%ld temp:%u.%d flags:%sx Q%su/%su%c\n"), target.X, target.Y, current_temp >> 2, (current_temp & 3) * 25, dda.allflags, mb_head, mb_tail, (queue_full()?'F':(queue_empty()?'E':' '))) \endcode
 */
 
-#ifdef SIMULATOR
-  #define GET_ARG(T) (va_arg(args, int))
-#else
+/**
+  va_arg() takes "fully promoted types" only, see example in Linux' va_arg
+  man page. This covers platforms >= 16 bits and arguments up to 32 bits.
+  64 bit arguments on a 32 bit platform will produce a severe warning.
+*/
+#if __SIZEOF_INT__ == 2
   #define GET_ARG(T) (va_arg(args, T))
+#elif __SIZEOF_INT__ >= 4
+  #define GET_ARG(T) ((T)va_arg(args, int))
 #endif
 
 void sersendf_P(PGM_P format_P, ...) {
@@ -130,7 +135,7 @@ void sersendf_P(PGM_P format_P, ...) {
 					j = 0;
 					break;
 				case 'c':
-          serial_writechar(GET_ARG(uint16_t));
+          serial_writechar((uint8_t)GET_ARG(uint16_t));
 					j = 0;
 					break;
 				case 'x':
diff --git a/sersendf.h b/sersendf.h
index 1ac2f90..a51760c 100644
--- a/sersendf.h
+++ b/sersendf.h
@@ -4,7 +4,8 @@
 #include "arduino.h"
 
 
-void sersendf(char *format, ...)		__attribute__ ((format (printf, 1, 2)));
-void sersendf_P(PGM_P format_P, ...)	__attribute__ ((format (printf, 1, 2)));
+// No __attribute__ ((format (printf, 1, 2)) here because %q isn't supported.
+void sersendf(char *format, ...);
+void sersendf_P(PGM_P format_P, ...);
 
 #endif	/* _SERSENDF_H */