diff --git a/promql/engine.go b/promql/engine.go
index 9f81b52a8..f7b2bbb8d 100644
--- a/promql/engine.go
+++ b/promql/engine.go
@@ -19,6 +19,7 @@ import (
 	"math"
 	"runtime"
 	"sort"
+	"strconv"
 	"time"
 
 	"golang.org/x/net/context"
@@ -43,12 +44,6 @@ type Sample struct {
 	Timestamp clientmodel.Timestamp   `json:"timestamp"`
 }
 
-// Scalar is a scalar value evaluated at the set timestamp.
-type Scalar struct {
-	Value     clientmodel.SampleValue `json:"value"`
-	Timestamp clientmodel.Timestamp   `json:"timestamp"`
-}
-
 // MarshalJSON implements json.Marshaler.
 func (s *Sample) MarshalJSON() ([]byte, error) {
 	v := struct {
@@ -65,16 +60,33 @@ func (s *Sample) MarshalJSON() ([]byte, error) {
 	return json.Marshal(&v)
 }
 
+// Scalar is a scalar value evaluated at the set timestamp.
+type Scalar struct {
+	Value     clientmodel.SampleValue `json:"value"`
+	Timestamp clientmodel.Timestamp   `json:"timestamp"`
+}
+
 func (s *Scalar) String() string {
 	return fmt.Sprintf("scalar: %v @[%v]", s.Value, s.Timestamp)
 }
 
+// MarshalJSON implements json.Marshaler.
+func (s *Scalar) MarshalJSON() ([]byte, error) {
+	v := strconv.FormatFloat(float64(s.Value), 'f', -1, 64)
+	return json.Marshal([]interface{}{s.Timestamp, string(v)})
+}
+
 // String is a string value evaluated at the set timestamp.
 type String struct {
 	Value     string                `json:"value"`
 	Timestamp clientmodel.Timestamp `json:"timestamp"`
 }
 
+// MarshalJSON implements json.Marshaler.
+func (s *String) MarshalJSON() ([]byte, error) {
+	return json.Marshal([]interface{}{s.Timestamp, s.Value})
+}
+
 func (s *String) String() string {
 	return s.Value
 }
diff --git a/web/api/legacy/query.go b/web/api/legacy/query.go
index de7279005..511d75bc8 100644
--- a/web/api/legacy/query.go
+++ b/web/api/legacy/query.go
@@ -94,11 +94,19 @@ func (api *API) Query(w http.ResponseWriter, r *http.Request) {
 		respondJSON(w, plainVec(vec))
 		return
 	}
+	if sca, ok := res.Value.(*promql.Scalar); ok {
+		respondJSON(w, (*plainScalar)(sca))
+		return
+	}
+	if str, ok := res.Value.(*promql.String); ok {
+		respondJSON(w, (*plainString)(str))
+		return
+	}
 
 	respondJSON(w, res.Value)
 }
 
-// plainVec is an indirection that hides the original MarshalJSON methods
+// plainVec is an indirection that hides the original MarshalJSON method
 // which does not fit the response format for the legacy API.
 type plainVec promql.Vector
 
@@ -121,6 +129,30 @@ func (pv plainVec) String() string {
 	return ""
 }
 
+// plainScalar is an indirection that hides the original MarshalJSON method
+// which does not fit the response format for the legacy API.
+type plainScalar promql.Scalar
+
+func (pv plainScalar) Type() promql.ExprType {
+	return promql.ExprScalar
+}
+
+func (pv plainScalar) String() string {
+	return ""
+}
+
+// plainString is an indirection that hides the original MarshalJSON method
+// which does not fit the response format for the legacy API.
+type plainString promql.String
+
+func (pv plainString) Type() promql.ExprType {
+	return promql.ExprString
+}
+
+func (pv plainString) String() string {
+	return ""
+}
+
 // QueryRange handles the /api/query_range endpoint.
 func (api *API) QueryRange(w http.ResponseWriter, r *http.Request) {
 	setAccessControlHeaders(w)