diff --git a/Modbus.h b/Modbus.h
index 72727a2..40055bc 100644
--- a/Modbus.h
+++ b/Modbus.h
@@ -31,7 +31,7 @@
 #define BIT_VAL(v) (v ? 0xFF00 : 0x0000)
 #define BIT_BOOL(v) (v == 0xFF00)
 #define COIL_VAL(v) (v ? 0xFF00 : 0x0000)
-#define COIL_BOOL(v) (v == 0xFF00)
+#define COIL_BOOL(v) (v == 0xFF00U)
 #define ISTS_VAL(v) (v ? 0xFF00 : 0x0000)
 #define ISTS_BOOL(v) (v == 0xFF00)