diff --git a/cache.go b/cache.go
index 7a400cf0f..9c909ced4 100644
--- a/cache.go
+++ b/cache.go
@@ -314,7 +314,7 @@ func (p BitmapPairs) Less(i, j int) bool { return p[i].Count > p[j].Count }
 // Pair holds an id/count pair.
 type Pair struct {
 	ID    uint64 `json:"id"`
-	Key   string `json:"key"`
+	Key   string `json:"key,omitempty"`
 	Count uint64 `json:"count"`
 }
 
diff --git a/handler_test.go b/handler_test.go
index 468d73a0e..f865d1510 100644
--- a/handler_test.go
+++ b/handler_test.go
@@ -541,7 +541,7 @@ func TestHandler_Query_Pairs_JSON(t *testing.T) {
 	h.ServeHTTP(w, test.MustNewHTTPRequest("POST", "/index/i/query", strings.NewReader(`TopN(frame=x, n=2)`)))
 	if w.Code != http.StatusOK {
 		t.Fatalf("unexpected status code: %d", w.Code)
-	} else if body := w.Body.String(); body != `{"results":[[{"id":1,"key":"","count":2},{"id":3,"key":"","count":4}]]}`+"\n" {
+	} else if body := w.Body.String(); body != `{"results":[[{"id":1,"count":2},{"id":3,"count":4}]]}`+"\n" {
 		t.Fatalf("unexpected body: %q", body)
 	}
 }