diff --git a/CHANGELOG.md b/CHANGELOG.md
index ee9a6dd775..efc9ba06e0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,9 @@
+## 2.4.3 / 2018-10-04
+
+* [BUGFIX] Fix panic when using custom EC2 API for SD #4672
+* [BUGFIX] Fix panic when Zookeeper SD cannot connect to servers #4669
+* [BUGFIX] Make the skip_head an optional parameter for snapshot API #4674
+
 ## 2.4.2 / 2018-09-21
 
  The last release didn't have bugfix included due to a vendoring error.
diff --git a/VERSION b/VERSION
index 8e8299dcc0..35cee72dcb 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.4.2
+2.4.3
diff --git a/discovery/ec2/ec2.go b/discovery/ec2/ec2.go
index d984fd3c66..a4f09cf068 100644
--- a/discovery/ec2/ec2.go
+++ b/discovery/ec2/ec2.go
@@ -241,9 +241,13 @@ func (d *Discovery) refresh() (tg *targetgroup.Group, err error) {
 					continue
 				}
 				labels := model.LabelSet{
-					ec2LabelOwnerID:    model.LabelValue(*r.OwnerId),
 					ec2LabelInstanceID: model.LabelValue(*inst.InstanceId),
 				}
+
+				if r.OwnerId != nil {
+					labels[ec2LabelOwnerID] = model.LabelValue(*r.OwnerId)
+				}
+
 				labels[ec2LabelPrivateIP] = model.LabelValue(*inst.PrivateIpAddress)
 				addr := net.JoinHostPort(*inst.PrivateIpAddress, fmt.Sprintf("%d", d.port))
 				labels[model.AddressLabel] = model.LabelValue(addr)
diff --git a/discovery/manager.go b/discovery/manager.go
index 0baa146502..8f98290d1b 100644
--- a/discovery/manager.go
+++ b/discovery/manager.go
@@ -345,12 +345,12 @@ func (m *Manager) registerProviders(cfg sd_config.ServiceDiscoveryConfig, setNam
 	}
 	for _, c := range cfg.ServersetSDConfigs {
 		add(c, func() (Discoverer, error) {
-			return zookeeper.NewServersetDiscovery(c, log.With(m.logger, "discovery", "zookeeper")), nil
+			return zookeeper.NewServersetDiscovery(c, log.With(m.logger, "discovery", "zookeeper"))
 		})
 	}
 	for _, c := range cfg.NerveSDConfigs {
 		add(c, func() (Discoverer, error) {
-			return zookeeper.NewNerveDiscovery(c, log.With(m.logger, "discovery", "nerve")), nil
+			return zookeeper.NewNerveDiscovery(c, log.With(m.logger, "discovery", "nerve"))
 		})
 	}
 	for _, c := range cfg.EC2SDConfigs {
diff --git a/discovery/zookeeper/zookeeper.go b/discovery/zookeeper/zookeeper.go
index 4871214576..7b5c1afc0c 100644
--- a/discovery/zookeeper/zookeeper.go
+++ b/discovery/zookeeper/zookeeper.go
@@ -115,12 +115,12 @@ type Discovery struct {
 }
 
 // NewNerveDiscovery returns a new Discovery for the given Nerve config.
-func NewNerveDiscovery(conf *NerveSDConfig, logger log.Logger) *Discovery {
+func NewNerveDiscovery(conf *NerveSDConfig, logger log.Logger) (*Discovery, error) {
 	return NewDiscovery(conf.Servers, time.Duration(conf.Timeout), conf.Paths, logger, parseNerveMember)
 }
 
 // NewServersetDiscovery returns a new Discovery for the given serverset config.
-func NewServersetDiscovery(conf *ServersetSDConfig, logger log.Logger) *Discovery {
+func NewServersetDiscovery(conf *ServersetSDConfig, logger log.Logger) (*Discovery, error) {
 	return NewDiscovery(conf.Servers, time.Duration(conf.Timeout), conf.Paths, logger, parseServersetMember)
 }
 
@@ -132,7 +132,7 @@ func NewDiscovery(
 	paths []string,
 	logger log.Logger,
 	pf func(data []byte, path string) (model.LabelSet, error),
-) *Discovery {
+) (*Discovery, error) {
 	if logger == nil {
 		logger = log.NewNopLogger()
 	}
@@ -143,7 +143,7 @@ func NewDiscovery(
 			c.SetLogger(treecache.NewZookeeperLogger(logger))
 		})
 	if err != nil {
-		return nil
+		return nil, err
 	}
 	updates := make(chan treecache.ZookeeperTreeCacheEvent)
 	sd := &Discovery{
@@ -156,7 +156,7 @@ func NewDiscovery(
 	for _, path := range paths {
 		sd.treeCaches = append(sd.treeCaches, treecache.NewZookeeperTreeCache(conn, path, updates, logger))
 	}
-	return sd
+	return sd, nil
 }
 
 // Run implements the Discoverer interface.
diff --git a/discovery/zookeeper/zookeeper_test.go b/discovery/zookeeper/zookeeper_test.go
new file mode 100644
index 0000000000..8900b2ab6f
--- /dev/null
+++ b/discovery/zookeeper/zookeeper_test.go
@@ -0,0 +1,32 @@
+// Copyright 2018 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package zookeeper
+
+import (
+	"testing"
+	"time"
+
+	"github.com/prometheus/common/model"
+)
+
+func TestNewDiscoveryError(t *testing.T) {
+	_, err := NewDiscovery(
+		[]string{"unreachable.test"},
+		time.Second, []string{"/"},
+		nil,
+		func(data []byte, path string) (model.LabelSet, error) { return nil, nil })
+	if err == nil {
+		t.Fatalf("expected error, got nil")
+	}
+}
diff --git a/web/api/v1/api.go b/web/api/v1/api.go
index d8a0eaaadc..aee136e870 100644
--- a/web/api/v1/api.go
+++ b/web/api/v1/api.go
@@ -926,9 +926,15 @@ func (api *API) snapshot(r *http.Request) (interface{}, *apiError, func()) {
 	if !api.enableAdmin {
 		return nil, &apiError{errorUnavailable, errors.New("Admin APIs disabled")}, nil
 	}
-	skipHead, err := strconv.ParseBool(r.FormValue("skip_head"))
-	if err != nil {
-		return nil, &apiError{errorUnavailable, fmt.Errorf("unable to parse boolean 'skip_head' argument: %v", err)}, nil
+	var (
+		skipHead bool
+		err      error
+	)
+	if r.FormValue("skip_head") != "" {
+		skipHead, err = strconv.ParseBool(r.FormValue("skip_head"))
+		if err != nil {
+			return nil, &apiError{errorUnavailable, fmt.Errorf("unable to parse boolean 'skip_head' argument: %v", err)}, nil
+		}
 	}
 
 	db := api.db()