diff --git a/src/groupSort.js b/src/groupSort.js
index 2d56ccf..63aba28 100644
--- a/src/groupSort.js
+++ b/src/groupSort.js
@@ -1,10 +1,9 @@
-import ascending from "./ascending.js";
 import group, {rollup} from "./group.js";
-import sort from "./sort.js";
+import sort, {ascendingDefined} from "./sort.js";
 
 export default function groupSort(values, reduce, key) {
   return (reduce.length !== 2
-    ? sort(rollup(values, reduce, key), (([ak, av], [bk, bv]) => ascending(av, bv) || ascending(ak, bk)))
-    : sort(group(values, key), (([ak, av], [bk, bv]) => reduce(av, bv) || ascending(ak, bk))))
+    ? sort(rollup(values, reduce, key), (([ak, av], [bk, bv]) => ascendingDefined(av, bv) || ascendingDefined(ak, bk)))
+    : sort(group(values, key), (([ak, av], [bk, bv]) => reduce(av, bv) || ascendingDefined(ak, bk))))
     .map(([key]) => key);
 }
diff --git a/test/groupSort-test.js b/test/groupSort-test.js
index 2d7d76b..6260d4f 100644
--- a/test/groupSort-test.js
+++ b/test/groupSort-test.js
@@ -31,6 +31,11 @@ it("groupSort(data, reduce, key) returns sorted keys when reduce is an accessor"
   );
 });
 
+it("groupSort(data, reduce, key) puts non-orderable reduced values last", () => {
+  const data = [{key: "z", value: 2}, {key: "a", value: null}, {key: "m", value: 1}];
+  assert.deepStrictEqual(groupSort(data, g => g[0].value, d => d.key), ["m", "z", "a"]);
+});
+
 it("groupSort(data, reduce, key) returns sorted keys when reduce is a comparator", () => {
   assert.deepStrictEqual(
     groupSort(barley, (a, b) => ascending(median(a, d => d.yield), median(b, d => d.yield)), d => d.variety),