diff --git a/prometheus/counter.go b/prometheus/counter.go
index 2f87690..03965f9 100644
--- a/prometheus/counter.go
+++ b/prometheus/counter.go
@@ -15,6 +15,7 @@ package prometheus
 
 import (
 	"errors"
+	"fmt"
 	"math"
 	"sync/atomic"
 
diff --git a/prometheus/gauge.go b/prometheus/gauge.go
index f335458..6b0ac42 100644
--- a/prometheus/gauge.go
+++ b/prometheus/gauge.go
@@ -14,6 +14,7 @@
 package prometheus
 
 import (
+	"fmt"
 	"math"
 	"sync/atomic"
 	"time"