diff --git a/src/command.c b/src/command.c
index 372b8e9..12a5e4a 100644
--- a/src/command.c
+++ b/src/command.c
@@ -357,11 +357,7 @@ static bool cmd_target_power(target *t, int argc, const char **argv)
 #ifdef PLATFORM_HAS_TRACESWO
 static bool cmd_traceswo(target *t, int argc, const char **argv)
 {
-#if defined(STM32L0) || defined(STM32F3) || defined(STM32F4)
-	extern char serial_no[13];
-#else
-	extern char serial_no[9];
-#endif
+	extern char *serial_no;
 	(void)t;
 #if TRACESWO_PROTOCOL == 2
 	uint32_t baudrate = SWO_DEFAULT_BAUD;