diff --git a/api/prometheus/v1/api.go b/api/prometheus/v1/api.go
index 9f9556676..96e98ea5b 100644
--- a/api/prometheus/v1/api.go
+++ b/api/prometheus/v1/api.go
@@ -1139,8 +1139,12 @@ func (h *httpAPI) Series(ctx context.Context, matches []string, startTime, endTi
 		q.Add("match[]", m)
 	}
 
-	q.Set("start", formatTime(startTime))
-	q.Set("end", formatTime(endTime))
+	if !startTime.IsZero() {
+		q.Set("start", formatTime(startTime))
+	}
+	if !endTime.IsZero() {
+		q.Set("end", formatTime(endTime))
+	}
 
 	u.RawQuery = q.Encode()