diff --git a/h/build.lua b/h/build.lua
index 4163e8249..93b1715d8 100644
--- a/h/build.lua
+++ b/h/build.lua
@@ -3,7 +3,7 @@ normalrule {
 	ins = {},
 	outleaves = { "em_path.h" },
 	commands = {
-		"echo '#define EM_DIR \"$(PREFIX)\"' >> %{outs}",
+		"echo '#define EM_DIR \"$(PREFIX)\"' > %{outs}",
 		"echo '#define ACK_PATH \"share/ack/descr\"' >> %{outs}",
 	}
 }