diff --git a/Godeps/_workspace/src/github.com/prometheus/client_golang/model/labelname.go b/Godeps/_workspace/src/github.com/prometheus/client_golang/model/labelname.go
index 7efbd9f41..ed6915ce8 100644
--- a/Godeps/_workspace/src/github.com/prometheus/client_golang/model/labelname.go
+++ b/Godeps/_workspace/src/github.com/prometheus/client_golang/model/labelname.go
@@ -14,6 +14,7 @@
 package model
 
 import (
+	"encoding/json"
 	"fmt"
 	"regexp"
 	"strings"
@@ -81,6 +82,19 @@ func (ln *LabelName) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	return nil
 }
 
+// UnmarshalJSON implements the json.Unmarshaler interface.
+func (ln *LabelName) UnmarshalJSON(b []byte) error {
+	var s string
+	if err := json.Unmarshal(b, &s); err != nil {
+		return err
+	}
+	if !LabelNameRE.MatchString(s) {
+		return fmt.Errorf("%q is not a valid label name", s)
+	}
+	*ln = LabelName(s)
+	return nil
+}
+
 // LabelNames is a sortable LabelName slice. In implements sort.Interface.
 type LabelNames []LabelName
 
diff --git a/Godeps/_workspace/src/github.com/prometheus/client_golang/model/labelset.go b/Godeps/_workspace/src/github.com/prometheus/client_golang/model/labelset.go
index b1b54fba3..d04d84320 100644
--- a/Godeps/_workspace/src/github.com/prometheus/client_golang/model/labelset.go
+++ b/Godeps/_workspace/src/github.com/prometheus/client_golang/model/labelset.go
@@ -14,6 +14,7 @@
 package model
 
 import (
+	"encoding/json"
 	"fmt"
 	"sort"
 	"strings"
@@ -62,3 +63,21 @@ func (l LabelSet) MergeFromMetric(m Metric) {
 		l[k] = v
 	}
 }
+
+// UnmarshalJSON implements the json.Unmarshaler interface.
+func (l *LabelSet) UnmarshalJSON(b []byte) error {
+	var m map[LabelName]LabelValue
+	if err := json.Unmarshal(b, &m); err != nil {
+		return err
+	}
+	// encoding/json only unmarshals maps of the form map[string]T. It does not
+	// detect that LabelName is a string and does not call its UnmarshalJSON method.
+	// Thus we have to replicate the behavior here.
+	for ln := range m {
+		if !LabelNameRE.MatchString(string(ln)) {
+			return fmt.Errorf("%q is not a valid label name", ln)
+		}
+	}
+	*l = LabelSet(m)
+	return nil
+}
diff --git a/config/config_test.go b/config/config_test.go
index 61aeb7f84..f907eb729 100644
--- a/config/config_test.go
+++ b/config/config_test.go
@@ -1,6 +1,8 @@
 package config
 
 import (
+	"encoding/json"
+	"io/ioutil"
 	"reflect"
 	"regexp"
 	"strings"
@@ -149,6 +151,9 @@ var expectedErrors = []struct {
 	}, {
 		filename: "labelname.bad.yml",
 		errMsg:   `"not$allowed" is not a valid label name`,
+	}, {
+		filename: "labelname2.bad.yml",
+		errMsg:   `"not:allowed" is not a valid label name`,
 	}, {
 		filename: "regex.bad.yml",
 		errMsg:   "error parsing regexp",
@@ -170,3 +175,15 @@ func TestBadConfigs(t *testing.T) {
 		}
 	}
 }
+
+func TestBadTargetGroup(t *testing.T) {
+	content, err := ioutil.ReadFile("testdata/tgroup.bad.json")
+	if err != nil {
+		t.Fatal(err)
+	}
+	var tg TargetGroup
+	err = json.Unmarshal(content, &tg)
+	if err == nil {
+		t.Errorf("Expected unmarshal error but got none.")
+	}
+}
diff --git a/config/testdata/labelname2.bad.yml b/config/testdata/labelname2.bad.yml
new file mode 100644
index 000000000..30da44b17
--- /dev/null
+++ b/config/testdata/labelname2.bad.yml
@@ -0,0 +1,3 @@
+global:
+  labels:
+    'not:allowed': value
diff --git a/config/testdata/tgroup.bad.json b/config/testdata/tgroup.bad.json
new file mode 100644
index 000000000..6050ed9c5
--- /dev/null
+++ b/config/testdata/tgroup.bad.json
@@ -0,0 +1,7 @@
+{
+  "targets": ["1.2.3.4:9100"],
+  "labels": {
+    "some_valid_label": "foo",
+    "oops:this-label-is-invalid": "bar"
+  }
+}