diff --git a/makefile b/makefile
index 5cba90e..287ffc6 100644
--- a/makefile
+++ b/makefile
@@ -2,7 +2,7 @@ EXECUTABLE = fm_transmitter
 VERSION = 0.9.6
 FLAGS = -Wall -O3 -std=c++11
 TRANSMITTER = -fno-strict-aliasing -I/opt/vc/include
-ifeq ($(GPIO21), 1)
+ifdef GPIO21
 	TRANSMITTER += -DGPIO21
 endif