diff --git a/middleware.go b/middleware.go
index e65c68b..e7914fe 100644
--- a/middleware.go
+++ b/middleware.go
@@ -122,19 +122,11 @@ type PrometheusPushGateway struct {
 }
 
 // NewPrometheus generates a new set of metrics with a certain subsystem name
-func NewPrometheus(subsystem string, customMetricsList ...[]*Metric) *Prometheus {
+func NewPrometheus(subsystem string, customMetricsList ...*Metric) *Prometheus {
 
-	var metricsList []*Metric
-
-	if len(customMetricsList) > 1 {
-		panic("Too many args. NewPrometheus( string, <optional []*Metric> ).")
-	} else if len(customMetricsList) == 1 {
-		metricsList = customMetricsList[0]
-	}
-
-	for _, metric := range standardMetrics {
-		metricsList = append(metricsList, metric)
-	}
+	metricsList := make([]*Metric, 0, len(customMetricsList)+len(standardMetrics))
+	metricsList = append(metricsList, customMetricsList...)
+	metricsList = append(metricsList, standardMetrics...)
 
 	p := &Prometheus{
 		MetricsList: metricsList,