diff --git a/birdvisu/topo_v3.py b/birdvisu/topo_v3.py
index 32126ab..b33c782 100644
--- a/birdvisu/topo_v3.py
+++ b/birdvisu/topo_v3.py
@@ -89,14 +89,14 @@ class TopologyV3:
 		# the Edge objects are not directly linked to Topology; to make the
 		# connection, one needs to resolve one of the VertexIDs to the specific
 		# Vertex to see if the edge is present.
-		self.vertices[source].outgoing_edges.add(edge)
-		self.vertices[target].incoming_edges.add(edge)
+		self.vertices[edge.source].outgoing_edges.add(edge)
+		self.vertices[edge.target].incoming_edges.add(edge)
 		self.edges.add(edge)
 
 	@classmethod
 	def combine_topologies(cls, **topologies) -> 'TopologyV3':
 		result = cls()
-		for label, topo in topologies:
+		for label, topo in topologies.items():
 			result.add_ancestor(label, topo)
 		return result