diff --git a/libcni/api_test.go b/libcni/api_test.go
index 0616bf4e0..97d003250 100644
--- a/libcni/api_test.go
+++ b/libcni/api_test.go
@@ -300,7 +300,8 @@ var _ = Describe("Invoking plugins", func() {
 					CNIVersion: current.ImplementedSpecVersion,
 					IPs: []*current.IPConfig{
 						{
-							Version: "4",
+							Version:   "4",
+							Interface: -1,
 							Address: net.IPNet{
 								IP:   net.ParseIP("10.1.2.3"),
 								Mask: net.IPv4Mask(255, 255, 255, 0),
@@ -484,7 +485,8 @@ var _ = Describe("Invoking plugins", func() {
 					// IP4 added by first plugin
 					IPs: []*current.IPConfig{
 						{
-							Version: "4",
+							Version:   "4",
+							Interface: -1,
 							Address: net.IPNet{
 								IP:   net.ParseIP("10.1.2.3"),
 								Mask: net.IPv4Mask(255, 255, 255, 0),
diff --git a/pkg/types/current/types.go b/pkg/types/current/types.go
index b5715fe66..5f0455544 100644
--- a/pkg/types/current/types.go
+++ b/pkg/types/current/types.go
@@ -70,10 +70,9 @@ func convertFrom020(result types.Result) (*Result, error) {
 
 	if oldResult.IP4 != nil {
 		newResult.IPs = append(newResult.IPs, &IPConfig{
-			Version:   "4",
-			Interface: -1,
-			Address:   oldResult.IP4.IP,
-			Gateway:   oldResult.IP4.Gateway,
+			Version: "4",
+			Address: oldResult.IP4.IP,
+			Gateway: oldResult.IP4.Gateway,
 		})
 		for _, route := range oldResult.IP4.Routes {
 			gw := route.GW
@@ -89,10 +88,9 @@ func convertFrom020(result types.Result) (*Result, error) {
 
 	if oldResult.IP6 != nil {
 		newResult.IPs = append(newResult.IPs, &IPConfig{
-			Version:   "6",
-			Interface: -1,
-			Address:   oldResult.IP6.IP,
-			Gateway:   oldResult.IP6.Gateway,
+			Version: "6",
+			Address: oldResult.IP6.IP,
+			Gateway: oldResult.IP6.Gateway,
 		})
 		for _, route := range oldResult.IP6.Routes {
 			gw := route.GW
@@ -253,7 +251,7 @@ func (i *Interface) String() string {
 type IPConfig struct {
 	// IP version, either "4" or "6"
 	Version string
-	// Index into Result structs Interfaces list
+	// Index into Result structs Interfaces list; -1 if no interface is given
 	Interface int
 	Address   net.IPNet
 	Gateway   net.IP
@@ -266,17 +264,19 @@ func (i *IPConfig) String() string {
 // JSON (un)marshallable types
 type ipConfig struct {
 	Version   string      `json:"version"`
-	Interface int         `json:"interface,omitempty"`
+	Interface *int        `json:"interface,omitempty"`
 	Address   types.IPNet `json:"address"`
 	Gateway   net.IP      `json:"gateway,omitempty"`
 }
 
 func (c *IPConfig) MarshalJSON() ([]byte, error) {
 	ipc := ipConfig{
-		Version:   c.Version,
-		Interface: c.Interface,
-		Address:   types.IPNet(c.Address),
-		Gateway:   c.Gateway,
+		Version: c.Version,
+		Address: types.IPNet(c.Address),
+		Gateway: c.Gateway,
+	}
+	if c.Interface >= 0 {
+		ipc.Interface = &c.Interface
 	}
 
 	return json.Marshal(ipc)
@@ -289,7 +289,10 @@ func (c *IPConfig) UnmarshalJSON(data []byte) error {
 	}
 
 	c.Version = ipc.Version
-	c.Interface = ipc.Interface
+	c.Interface = -1
+	if ipc.Interface != nil {
+		c.Interface = *ipc.Interface
+	}
 	c.Address = net.IPNet(ipc.Address)
 	c.Gateway = ipc.Gateway
 	return nil
diff --git a/pkg/types/current/types_test.go b/pkg/types/current/types_test.go
index afc68670a..c73fe9b18 100644
--- a/pkg/types/current/types_test.go
+++ b/pkg/types/current/types_test.go
@@ -15,6 +15,7 @@
 package current_test
 
 import (
+	"encoding/json"
 	"io/ioutil"
 	"net"
 	"os"
@@ -103,7 +104,7 @@ var _ = Describe("Current types operations", func() {
 		os.Stdout = oldStdout
 		Expect(err).NotTo(HaveOccurred())
 
-		Expect(string(out)).To(Equal(`{
+		Expect(string(out)).To(MatchJSON(`{
     "cniVersion": "0.3.1",
     "interfaces": [
         {
@@ -115,11 +116,13 @@ var _ = Describe("Current types operations", func() {
     "ips": [
         {
             "version": "4",
+            "interface": 0,
             "address": "1.2.3.30/24",
             "gateway": "1.2.3.1"
         },
         {
             "version": "6",
+            "interface": 0,
             "address": "abcd:1234:ffff::cdde/64",
             "gateway": "abcd:1234:ffff::1"
         }
@@ -173,7 +176,7 @@ var _ = Describe("Current types operations", func() {
 		os.Stdout = oldStdout
 		Expect(err).NotTo(HaveOccurred())
 
-		Expect(string(out)).To(Equal(`{
+		Expect(string(out)).To(MatchJSON(`{
     "cniVersion": "0.2.0",
     "ip4": {
         "ip": "1.2.3.30/24",
@@ -210,6 +213,43 @@ var _ = Describe("Current types operations", func() {
             "bar"
         ]
     }
+}`))
+	})
+
+	It("correctly marshals interface index 0", func() {
+		ipc := &current.IPConfig{
+			Version:   "4",
+			Interface: 0,
+			Address: net.IPNet{
+				IP:   net.ParseIP("10.1.2.3"),
+				Mask: net.IPv4Mask(255, 255, 255, 0),
+			},
+		}
+
+		json, err := json.Marshal(ipc)
+		Expect(err).NotTo(HaveOccurred())
+		Expect(json).To(MatchJSON(`{
+    "version": "4",
+    "interface": 0,
+    "address": "10.1.2.3/24"
+}`))
+	})
+
+	It("correctly marshals a missing interface index", func() {
+		ipc := &current.IPConfig{
+			Version:   "4",
+			Interface: -1,
+			Address: net.IPNet{
+				IP:   net.ParseIP("10.1.2.3"),
+				Mask: net.IPv4Mask(255, 255, 255, 0),
+			},
+		}
+
+		json, err := json.Marshal(ipc)
+		Expect(err).NotTo(HaveOccurred())
+		Expect(json).To(MatchJSON(`{
+    "version": "4",
+    "address": "10.1.2.3/24"
 }`))
 	})
 })