diff --git a/http/client.go b/http/client.go
index 138dab6..7063808 100644
--- a/http/client.go
+++ b/http/client.go
@@ -127,7 +127,7 @@ func (c *Client) wrap(req *http.Request) error {
 		length := make([]byte, 4)
 		binary.LittleEndian.PutUint32(length, uint32(len(signature)))
 
-		body, newContentType, err := Wrap(concat(length, signature, sealed), contentType)
+		body, newContentType, err := Wrap(concat(length, signature, sealed), contentType, len(body))
 		if err != nil {
 			return err
 		}
diff --git a/http/mime.go b/http/mime.go
index e044baa..d996cca 100644
--- a/http/mime.go
+++ b/http/mime.go
@@ -98,7 +98,7 @@ func fromWSMV(r io.Reader) io.Reader {
 	})
 }
 
-func Wrap(input []byte, ct string) ([]byte, string, error) {
+func Wrap(input []byte, ct string, contentLength int) ([]byte, string, error) {
 	b := &bytes.Buffer{}
 
 	writer := multipart.NewWriter(b)
@@ -109,7 +109,7 @@ func Wrap(input []byte, ct string) ([]byte, string, error) {
 	header := make(textproto.MIMEHeader)
 	header.Set(contentTypeHeader, mimeProtocol)
 	// Using Set() will canonicalise this to "Originalcontent" which may cause problems
-	header["OriginalContent"] = []string{fmt.Sprintf("type=%s;Length=%d", ct, len(input))}
+	header["OriginalContent"] = []string{fmt.Sprintf("type=%s;Length=%d", ct, contentLength)}
 
 	if _, err := writer.CreatePart(header); err != nil {
 		return nil, "", err