diff --git a/config/config.go b/config/config.go
index d2b5a9a..6dedc2b 100644
--- a/config/config.go
+++ b/config/config.go
@@ -90,10 +90,20 @@ func IsZero(c interface{}) bool {
 	return isZero(reflect.ValueOf(c))
 }
 
-// AssertValid checks the fields in the structure and makes sure that they
-// contain valid values as specified by the 'valid' flag. Empty fields are
+type ErrorValid struct {
+	Value string
+	Valid []string
+	Field string
+}
+
+func (e ErrorValid) Error() string {
+	return fmt.Sprintf("invalid value %q for option %q (valid options: %q)", e.Value, e.Field, e.Valid)
+}
+
+// AssertStructValid checks the fields in the structure and makes sure that
+// they contain valid values as specified by the 'valid' flag. Empty fields are
 // implicitly valid.
-func AssertValid(c interface{}) error {
+func AssertStructValid(c interface{}) error {
 	ct := reflect.TypeOf(c)
 	cv := reflect.ValueOf(c)
 	for i := 0; i < ct.NumField(); i++ {
@@ -102,15 +112,33 @@ func AssertValid(c interface{}) error {
 			continue
 		}
 
-		valid := ft.Tag.Get("valid")
-		val := cv.Field(i)
-		if !isValid(val, valid) {
-			return fmt.Errorf("invalid value \"%v\" for option %q (valid options: %q)", val.Interface(), ft.Name, valid)
+		if err := AssertValid(cv.Field(i), ft.Tag.Get("valid")); err != nil {
+			err.Field = ft.Name
+			return err
 		}
 	}
 	return nil
 }
 
+// AssertValid checks to make sure that the given value is in the list of
+// valid values. Zero values are implicitly valid.
+func AssertValid(value reflect.Value, valid string) *ErrorValid {
+	if valid == "" || isZero(value) {
+		return nil
+	}
+	vs := fmt.Sprintf("%v", value.Interface())
+	valids := strings.Split(valid, ",")
+	for _, valid := range valids {
+		if vs == valid {
+			return nil
+		}
+	}
+	return &ErrorValid{
+		Value: vs,
+		Valid: valids,
+	}
+}
+
 func isZero(v reflect.Value) bool {
 	switch v.Kind() {
 	case reflect.Struct:
@@ -130,19 +158,6 @@ func isFieldExported(f reflect.StructField) bool {
 	return f.PkgPath == ""
 }
 
-func isValid(v reflect.Value, valid string) bool {
-	if valid == "" || isZero(v) {
-		return true
-	}
-	vs := fmt.Sprintf("%v", v.Interface())
-	for _, valid := range strings.Split(valid, ",") {
-		if vs == valid {
-			return true
-		}
-	}
-	return false
-}
-
 type warner func(format string, v ...interface{})
 
 // warnOnUnrecognizedKeys parses the contents of a cloud-config file and calls
diff --git a/config/config_test.go b/config/config_test.go
index 1d776d6..ea68947 100644
--- a/config/config_test.go
+++ b/config/config_test.go
@@ -17,7 +17,6 @@
 package config
 
 import (
-	"errors"
 	"fmt"
 	"reflect"
 	"strings"
@@ -43,7 +42,7 @@ func TestIsZero(t *testing.T) {
 	}
 }
 
-func TestAssertValid(t *testing.T) {
+func TestAssertStructValid(t *testing.T) {
 	for _, tt := range []struct {
 		c   interface{}
 		err error
@@ -60,7 +59,7 @@ func TestAssertValid(t *testing.T) {
 		}{A: "1", b: "hello"}, nil},
 		{struct {
 			A, b string `valid:"1,2"`
-		}{A: "hello", b: "2"}, errors.New("invalid value \"hello\" for option \"A\" (valid options: \"1,2\")")},
+		}{A: "hello", b: "2"}, &ErrorValid{Value: "hello", Field: "A", Valid: []string{"1", "2"}}},
 		{struct {
 			A, b int `valid:"1,2"`
 		}{}, nil},
@@ -72,9 +71,9 @@ func TestAssertValid(t *testing.T) {
 		}{A: 1, b: 9}, nil},
 		{struct {
 			A, b int `valid:"1,2"`
-		}{A: 9, b: 2}, errors.New("invalid value \"9\" for option \"A\" (valid options: \"1,2\")")},
+		}{A: 9, b: 2}, &ErrorValid{Value: "9", Field: "A", Valid: []string{"1", "2"}}},
 	} {
-		if err := AssertValid(tt.c); !reflect.DeepEqual(tt.err, err) {
+		if err := AssertStructValid(tt.c); !reflect.DeepEqual(tt.err, err) {
 			t.Errorf("bad result (%q): want %q, got %q", tt.c, tt.err, err)
 		}
 	}
diff --git a/system/update.go b/system/update.go
index 2038def..5d6b066 100644
--- a/system/update.go
+++ b/system/update.go
@@ -61,7 +61,7 @@ func (uc Update) File() (*File, error) {
 	if config.IsZero(uc.Update) {
 		return nil, nil
 	}
-	if err := config.AssertValid(uc.Update); err != nil {
+	if err := config.AssertStructValid(uc.Update); err != nil {
 		return nil, err
 	}
 
diff --git a/system/update_test.go b/system/update_test.go
index a75348e..7b68172 100644
--- a/system/update_test.go
+++ b/system/update_test.go
@@ -17,7 +17,6 @@
 package system
 
 import (
-	"errors"
 	"io"
 	"reflect"
 	"strings"
@@ -101,7 +100,7 @@ func TestUpdateFile(t *testing.T) {
 		},
 		{
 			config: config.Update{RebootStrategy: "wizzlewazzle"},
-			err:    errors.New("invalid value \"wizzlewazzle\" for option \"RebootStrategy\" (valid options: \"best-effort,etcd-lock,reboot,false\")"),
+			err:    &config.ErrorValid{Value: "wizzlewazzle", Field: "RebootStrategy", Valid: []string{"best-effort", "etcd-lock", "reboot", "false"}},
 		},
 		{
 			config: config.Update{Group: "master", Server: "http://foo.com"},