diff --git a/header.go b/header.go
index 3c94f7d..c78b9a7 100644
--- a/header.go
+++ b/header.go
@@ -100,7 +100,7 @@ func headerWriteSubset(h http.Header, exclude map[string]bool, writeHeader func(
 		}
 		for i, v := range kv.Values {
 			vv := headerNewlineToSpace.Replace(v)
-			vv = textproto.TrimString(v)
+			vv = textproto.TrimString(vv)
 			if vv != v {
 				kv.Values[i] = vv
 			}