diff --git a/spectator-reg-atlas/src/main/java/com/netflix/spectator/atlas/impl/Evaluator.java b/spectator-reg-atlas/src/main/java/com/netflix/spectator/atlas/impl/Evaluator.java
index 9ed0e0e7c..252b1c10f 100644
--- a/spectator-reg-atlas/src/main/java/com/netflix/spectator/atlas/impl/Evaluator.java
+++ b/spectator-reg-atlas/src/main/java/com/netflix/spectator/atlas/impl/Evaluator.java
@@ -197,7 +197,7 @@ public EvalPayload eval(long timestamp) {
                 double acc = expr.isCount() ? 1.0 : v;
                 metrics.add(new EvalPayload.Metric(subId, tags, acc));
               }
-            } else {
+            } else if (tags != null) {
               TagsValuePair p = new TagsValuePair(tags, v);
               aggregator.update(p);
               LOGGER.trace("aggregating: {}: {}", timestamp, p);
diff --git a/spectator-reg-atlas/src/test/java/com/netflix/spectator/atlas/impl/EvaluatorTest.java b/spectator-reg-atlas/src/test/java/com/netflix/spectator/atlas/impl/EvaluatorTest.java
index 003978aae..cbd21d1b6 100644
--- a/spectator-reg-atlas/src/test/java/com/netflix/spectator/atlas/impl/EvaluatorTest.java
+++ b/spectator-reg-atlas/src/test/java/com/netflix/spectator/atlas/impl/EvaluatorTest.java
@@ -123,6 +123,19 @@ public void sumAndMaxForGroup() {
     Assertions.assertEquals(expected, sort(payload));
   }
 
+  @Test
+  public void missingTagGroupBy() {
+    List<Subscription> subs = new ArrayList<>();
+    subs.add(newSubscription("sum", ":true,:sum,(,app,),:by"));
+
+    Evaluator evaluator = newEvaluator();
+    evaluator.sync(subs);
+    EvalPayload payload = evaluator.eval(0L, data("foo", 1.0, 2.0, 3.0));
+
+    EvalPayload expected = new EvalPayload(0L, Collections.emptyList());
+    Assertions.assertEquals(expected, sort(payload));
+  }
+
   @Test
   public void updateSub() {