diff --git a/app/vmselect/promql/rollup.go b/app/vmselect/promql/rollup.go
index c711f04ad8..2685f72ca7 100644
--- a/app/vmselect/promql/rollup.go
+++ b/app/vmselect/promql/rollup.go
@@ -1142,10 +1142,10 @@ func rollupSum(rfa *rollupFuncArg) float64 {
 	// before calling rollup funcs.
 	values := rfa.values
 	if len(values) == 0 {
-		if math.IsNaN(rfa.prevValue) {
-			return nan
-		}
-		return 0
+		// Do not take into account rfa.prevValue, since it may lead
+		// to inconsistent results comparing to Prometheus on broken time series
+		// with irregular data points.
+		return nan
 	}
 	var sum float64
 	for _, v := range values {