diff --git a/.travis.yml b/.travis.yml
index 397d166..c4820d0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,6 @@
 language: go
 
 go:
-  - 1.2
   - 1.3
   - 1.4
   - 1.5
@@ -9,6 +8,10 @@ go:
   - 1.7
   - 1.8
   - 1.9
+  - "1.10"
+  - 1.11
+  - 1.12
+  - 1.13
   - master
 
 # safelist
diff --git a/message.go b/message.go
index 9f7f7bd..0993737 100644
--- a/message.go
+++ b/message.go
@@ -105,14 +105,16 @@ func (m *Message) SetBoundary(boundary string) {
 
 // SetHeader sets a value to the given header field.
 func (m *Message) SetHeader(field string, value ...string) {
-	m.encodeHeader(value)
-	m.header[field] = value
+	m.header[field] = m.encodeHeader(value)
 }
 
-func (m *Message) encodeHeader(values []string) {
+func (m *Message) encodeHeader(values []string) []string {
+	encoded := make([]string, len(values))
 	for i := range values {
-		values[i] = m.encodeString(values[i])
+		encoded[i] = m.encodeString(values[i])
 	}
+
+	return encoded
 }
 
 func (m *Message) encodeString(value string) string {