diff --git a/README.md b/README.md
index 0730399..00f3f56 100644
--- a/README.md
+++ b/README.md
@@ -240,10 +240,13 @@ You can implement your own unmarshalling of responses by implementing the `Unmar
 
 ```go
 type Unmarshaler interface {
-  Unmarshal(mimetype string, r io.Reader) error
+  Unmarshal(header http.Header, r io.Reader) error
 }
 ```
 
+The first argument to the `Unmarshal` method is the HTTP header of the response, and the second
+argument is the body of the response. The method should return an error if the unmarshalling fails.
+
 ## Text Streaming Responses
 
 The client implements a streaming text event callback which can be used to process a stream of text events,
diff --git a/client.go b/client.go
index 76e4334..587f328 100644
--- a/client.go
+++ b/client.go
@@ -24,7 +24,7 @@ import (
 // Unmarshaler is an interface which can be implemented by a type to
 // unmarshal a response body
 type Unmarshaler interface {
-	Unmarshal(mimetype string, r io.Reader) error
+	Unmarshal(header http.Header, r io.Reader) error
 }
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -333,7 +333,7 @@ func do(client *http.Client, req *http.Request, accept string, strict bool, out
 		}
 	default:
 		if v, ok := out.(Unmarshaler); ok {
-			return v.Unmarshal(mimetype, response.Body)
+			return v.Unmarshal(response.Header, response.Body)
 		} else if v, ok := out.(io.Writer); ok {
 			if _, err := io.Copy(v, response.Body); err != nil {
 				return err