diff --git a/datatype/labelmap/mutate.go b/datatype/labelmap/mutate.go
index 12a1ba09..0a21ca9c 100644
--- a/datatype/labelmap/mutate.go
+++ b/datatype/labelmap/mutate.go
@@ -105,6 +105,10 @@ func (d *Data) MergeLabels(v dvid.VersionID, op labels.MergeOp, info dvid.ModInf
 		return
 	}
 	delta.MergedVoxels = mergeIdx.NumVoxels()
+	if delta.MergedVoxels == 0 {
+		err = fmt.Errorf("can't merge labels with no voxels: %s", op.Merged)
+		return
+	}
 
 	jsonBytes, _ := json.Marshal(msginfo)
 	if err := d.PublishKafkaMsg(jsonBytes); err != nil {
diff --git a/datatype/labelmap/mutate_test.go b/datatype/labelmap/mutate_test.go
index 41c03881..36b2c6f4 100644
--- a/datatype/labelmap/mutate_test.go
+++ b/datatype/labelmap/mutate_test.go
@@ -945,6 +945,11 @@ func TestMergeLabels(t *testing.T) {
 	if err := retrieved.equals(expected); err != nil {
 		t.Errorf("Merged label volume: %v\n", err)
 	}
+
+	// Make sure we can't merge the previously merged body again.
+	if err := testMerge.sendErr(t, uuid, "labels"); err == nil {
+		t.Fatalf("Expected error on re-merge of bodies, got none\n")
+	}
 }
 
 func TestSplitLabel(t *testing.T) {