diff --git a/src/traits.rs b/src/traits.rs
index ffb2cc1..a767df0 100644
--- a/src/traits.rs
+++ b/src/traits.rs
@@ -5,7 +5,7 @@
 // TODO: Split out GraphTopologyAddition trait.
 pub trait GraphTopology {
     type Vertex: Copy + Eq;
-    type Edge;
+    type Edge: Copy + Eq;
 
     fn vertex_count(&self) -> usize;
     fn edge_count(&self) -> usize;