diff --git a/src/Data/Graph.hs b/src/Data/Graph.hs
index efa46a48c..ca1dd6bee 100644
--- a/src/Data/Graph.hs
+++ b/src/Data/Graph.hs
@@ -21,3 +21,7 @@ simplify (Graph graph) = Graph (G.simplify graph)
 
 instance Semigroup (Graph v) where
   (<>) = Class.overlay
+
+instance Monoid (Graph v) where
+  mempty = Class.empty
+  mappend = (<>)