diff --git a/2015/hou/Makefile b/2015/hou/Makefile
index 96f755400..c0b18a6ad 100644
--- a/2015/hou/Makefile
+++ b/2015/hou/Makefile
@@ -167,6 +167,7 @@ clean:
 
 clobber: clean
 	${RM} -f ${TARGET} ${ALT_TARGET}
+	${RM} -f prebuild
 	@-if [ -e sandwich ]; then \
 	    ${RM} -f sandwich; \
 	    echo 'ate sandwich'; \