diff --git a/libnetwork/ipams/null/null.go b/libnetwork/ipams/null/null.go
index f757e53a9d..91e6fcc2dc 100644
--- a/libnetwork/ipams/null/null.go
+++ b/libnetwork/ipams/null/null.go
@@ -3,27 +3,28 @@
 package null
 
 import (
-	"fmt"
 	"net"
 
 	"github.com/docker/docker/libnetwork/ipamapi"
 	"github.com/docker/docker/libnetwork/types"
 )
 
-var (
-	defaultAS      = "null"
-	defaultPool, _ = types.ParseCIDR("0.0.0.0/0")
-	defaultPoolID  = fmt.Sprintf("%s/%s", defaultAS, defaultPool.String())
+const (
+	defaultAddressSpace = "null"
+	defaultPoolCIDR     = "0.0.0.0/0"
+	defaultPoolID       = defaultAddressSpace + "/" + defaultPoolCIDR
 )
 
+var defaultPool, _ = types.ParseCIDR(defaultPoolCIDR)
+
 type allocator struct{}
 
 func (a *allocator) GetDefaultAddressSpaces() (string, string, error) {
-	return defaultAS, defaultAS, nil
+	return defaultAddressSpace, defaultAddressSpace, nil
 }
 
-func (a *allocator) RequestPool(addressSpace, pool, subPool string, options map[string]string, v6 bool) (string, *net.IPNet, map[string]string, error) {
-	if addressSpace != defaultAS {
+func (a *allocator) RequestPool(addressSpace, pool, subPool string, _ map[string]string, v6 bool) (string, *net.IPNet, map[string]string, error) {
+	if addressSpace != defaultAddressSpace {
 		return "", nil, nil, types.BadRequestErrorf("unknown address space: %s", addressSpace)
 	}
 	if pool != "" {
diff --git a/libnetwork/ipams/null/null_test.go b/libnetwork/ipams/null/null_test.go
index deb4bf3946..c055b288bc 100644
--- a/libnetwork/ipams/null/null_test.go
+++ b/libnetwork/ipams/null/null_test.go
@@ -9,7 +9,7 @@ import (
 func TestPoolRequest(t *testing.T) {
 	a := allocator{}
 
-	pid, pool, _, err := a.RequestPool(defaultAS, "", "", nil, false)
+	pid, pool, _, err := a.RequestPool(defaultAddressSpace, "", "", nil, false)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -25,17 +25,17 @@ func TestPoolRequest(t *testing.T) {
 		t.Fatal("Unexpected success")
 	}
 
-	_, _, _, err = a.RequestPool(defaultAS, "192.168.0.0/16", "", nil, false)
+	_, _, _, err = a.RequestPool(defaultAddressSpace, "192.168.0.0/16", "", nil, false)
 	if err == nil {
 		t.Fatal("Unexpected success")
 	}
 
-	_, _, _, err = a.RequestPool(defaultAS, "", "192.168.0.0/24", nil, false)
+	_, _, _, err = a.RequestPool(defaultAddressSpace, "", "192.168.0.0/24", nil, false)
 	if err == nil {
 		t.Fatal("Unexpected success")
 	}
 
-	_, _, _, err = a.RequestPool(defaultAS, "", "", nil, true)
+	_, _, _, err = a.RequestPool(defaultAddressSpace, "", "", nil, true)
 	if err == nil {
 		t.Fatal("Unexpected success")
 	}