diff --git a/src/message/mimebody.rs b/src/message/mimebody.rs
index 6d056e5..52a3c8a 100644
--- a/src/message/mimebody.rs
+++ b/src/message/mimebody.rs
@@ -209,7 +209,7 @@ fn make_boundary() -> String {
 }
 
 impl MultiPartKind {
-    fn to_mime<S: AsRef<str>>(self, boundary: Option<S>) -> Mime {
+    fn to_mime<S: AsRef<str>>(&self, boundary: Option<S>) -> Mime {
         let boundary = boundary
             .map(|s| s.as_ref().into())
             .unwrap_or_else(make_boundary);