diff --git a/graphql/client.go b/graphql/client.go
index db7b8269..f938b25f 100644
--- a/graphql/client.go
+++ b/graphql/client.go
@@ -47,7 +47,7 @@ type Client interface {
 }
 
 type client struct {
-	httpClient *http.Client
+	httpClient Doer
 	endpoint   string
 	method     string
 }
@@ -61,13 +61,20 @@ type client struct {
 //
 // The typical method of adding authentication headers is to wrap the client's
 // Transport to add those headers.  See example/caller.go for an example.
-func NewClient(endpoint string, httpClient *http.Client) Client {
-	if httpClient == nil {
+func NewClient(endpoint string, httpClient Doer) Client {
+	if httpClient == nil || httpClient == (*http.Client)(nil) {
 		httpClient = http.DefaultClient
 	}
 	return &client{httpClient, endpoint, http.MethodPost}
 }
 
+// Doer encapsulates the methods from *http.Client needed by Client.
+// The methods should have behavior to match that of *http.Client
+// (or mocks for the same).
+type Doer interface {
+	Do(*http.Request) (*http.Response, error)
+}
+
 type payload struct {
 	Query     string      `json:"query"`
 	Variables interface{} `json:"variables,omitempty"`