diff --git a/mk/os-prefix.mk b/mk/os-prefix.mk
index 94fda963..c546a862 100644
--- a/mk/os-prefix.mk
+++ b/mk/os-prefix.mk
@@ -1,6 +1,6 @@
 # Copyright (c) 2012 William Hubbs <w.d.hubbs@gmail.com>
 # Released under the 2-clause BSD license.
 
-ifeq (${MKPREFIX},yes}
+ifeq (${MKPREFIX},yes)
 CPPFLAGS+=	-DPREFIX
 endif