diff --git a/src/app/credExplorer/pagerankTable/aggregate.js b/src/app/credExplorer/pagerankTable/aggregate.js
index 1664e25..c2594e0 100644
--- a/src/app/credExplorer/pagerankTable/aggregate.js
+++ b/src/app/credExplorer/pagerankTable/aggregate.js
@@ -2,6 +2,7 @@
 
 import sortBy from "lodash.sortby";
 import stringify from "json-stable-stringify";
+import * as MapUtil from "../../../util/map";
 import {NodeTrie, EdgeTrie} from "../../../core/trie";
 import type {NodeType, EdgeType} from "../../adapters/pluginAdapter";
 import type {ScoredConnection} from "../../../core/attribution/pagerankNodeDecomposition";
@@ -53,11 +54,7 @@ export function aggregateByNodeType(
   const nodeTypeToConnections = new Map();
   for (const x of xs) {
     const type = typeTrie.getLast(x.source);
-    const connections = nodeTypeToConnections.get(type) || [];
-    if (connections.length === 0) {
-      nodeTypeToConnections.set(type, connections);
-    }
-    connections.push(x);
+    MapUtil.pushValue(nodeTypeToConnections, type, x);
   }
   const aggregations: NodeAggregation[] = [];
   for (const [
@@ -109,11 +106,7 @@ export function aggregateByConnectionType(
     }
     const edge = x.connection.adjacency.edge;
     const type = typeTrie.getLast(edge.address);
-    const connections = relevantMap.get(type) || [];
-    if (connections.length === 0) {
-      relevantMap.set(type, connections);
-    }
-    connections.push(x);
+    MapUtil.pushValue(relevantMap, type, x);
   }
 
   function createAggregation(
diff --git a/src/util/map.js b/src/util/map.js
index b4646d0..f0865c9 100644
--- a/src/util/map.js
+++ b/src/util/map.js
@@ -146,3 +146,19 @@ export function merge<K, V>(
   }
   return result;
 }
+
+/**
+ * Given a map whose values are arrays, push an element onto the array
+ * corresponding to the given key. If the key is not in the map, first
+ * insert it with value a new empty array.
+ *
+ * If the key is already in the map, its value will be mutated, not
+ * replaced.
+ */
+export function pushValue<K, V>(map: Map<K, V[]>, key: K, value: V): void {
+  let arr = map.get(key);
+  if (arr == null) {
+    map.set(key, (arr = []));
+  }
+  arr.push(value);
+}
diff --git a/src/util/map.test.js b/src/util/map.test.js
index 577fd2d..7c7ae50 100644
--- a/src/util/map.test.js
+++ b/src/util/map.test.js
@@ -294,4 +294,27 @@ describe("util/map", () => {
       ]);
     });
   });
+  describe("pushValue", () => {
+    it("works when the map has no matching key", () => {
+      const map = new Map();
+      MapUtil.pushValue(map, "foo", 3);
+      expect(map).toEqual(new Map().set("foo", [3]));
+    });
+    it("works when the map has a matching key", () => {
+      const map = new Map().set("foo", [3]);
+      MapUtil.pushValue(map, "foo", 4);
+      expect(map).toEqual(new Map().set("foo", [3, 4]));
+    });
+    it("works when the map already has an empty array", () => {
+      const map = new Map().set("foo", []);
+      MapUtil.pushValue(map, "foo", 1);
+      expect(map).toEqual(new Map().set("foo", [1]));
+    });
+    it("preserves array identity", () => {
+      const arr = [];
+      const map = new Map().set("foo", arr);
+      MapUtil.pushValue(map, "foo", 1);
+      expect(map.get("foo")).toBe(arr);
+    });
+  });
 });