diff --git a/doc/Makefile b/doc/Makefile
index 2abac1ce..a041f834 100644
--- a/doc/Makefile
+++ b/doc/Makefile
@@ -44,7 +44,7 @@ coveragetable:
 
 html: api coveragetable
 	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(DEST)/html
-	cp source/plots $(DEST)/html -r
+	cp -r source/plots $(DEST)/html
 	@echo
 	@echo "Build finished. The HTML pages are in build/html."