diff --git a/src/Data/Graph/AdjList.hs b/src/Data/Graph/AdjList.hs
index 43b5519f5..4a48a17f3 100644
--- a/src/Data/Graph/AdjList.hs
+++ b/src/Data/Graph/AdjList.hs
@@ -88,7 +88,7 @@ graphToAdjList = taggedGraphToAdjList . tagGraph
 -- Using a PBGraph as the accumulator for the fold would incur
 -- significant overhead associated with Vector concatenation.
 -- We use this and then pay the O(v + e) to-Vector cost once.
-data Acc = Acc ![Vertex] !(HashSet Edge)
+data Acc = Acc [Vertex] (HashSet Edge)
 
 -- Convert a graph with tagged members to a protobuf-compatible adjacency list.
 -- The Tag is necessary to build a canonical adjacency list.