diff --git a/tests/Makefile b/tests/Makefile
index 487eaf0..1fb9e56 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -7,4 +7,4 @@ CPPFLAGS = -Wno-write-strings -DSTB_DIVIDE_TEST
 all:
 	$(CC) $(INCLUDES) $(CFLAGS) ../stb_vorbis.c test_c_compilation.c test_c_lexer.c test_dxt.c test_easyfont.c test_image.c test_image_write.c test_perlin.c test_sprintf.c test_truetype.c test_voxel.c -lm
 	$(CC) $(INCLUDES) $(CPPFLAGS) test_cpp_compilation.cpp -lm -lstdc++
-	$(CC) $(INCLUDES) $(CFLAGS) image_write_test.c -lm -o image_write_test
+	$(CC) $(INCLUDES) $(CFLAGS) -DIWT_TEST image_write_test.c -lm -o image_write_test