diff --git a/esp8266/Makefile b/esp8266/Makefile
index 9a76027026..65093d819b 100644
--- a/esp8266/Makefile
+++ b/esp8266/Makefile
@@ -26,7 +26,7 @@ INC += -I$(BUILD)
 INC += -I$(ESP_SDK)/include
 
 # UART for "os" messages. 0 is normal UART as used by MicroPython REPL,
-# 1 is debug UART (tx only).
+# 1 is debug UART (tx only), -1 to disable.
 UART_OS = 0
 
 CFLAGS_XTENSA = -fsingle-precision-constant -Wdouble-promotion \