diff --git a/make-includes/build_lecture_rules.make b/make-includes/build_lecture_rules.make
index 6683873..715c530 100644
--- a/make-includes/build_lecture_rules.make
+++ b/make-includes/build_lecture_rules.make
@@ -344,12 +344,12 @@
 # Clean up: get rid of all the temporary files.
 #
 tidy:
-	$(RM) -f $(TIDY_FILES)
+	$(RM) -rf $(TIDY_FILES)
 #
 # Clean up: get rid of everything except the original source.
 #
 clean: tidy
-	$(RM) -f $(CLEAN_FILES)
+	$(RM) -rf $(CLEAN_FILES)
 
 
 ################################################################################