diff --git a/src/net/mail/message.go b/src/net/mail/message.go
index 985b6fcae2..5770e3d8dc 100644
--- a/src/net/mail/message.go
+++ b/src/net/mail/message.go
@@ -745,17 +745,41 @@ func (p *addrParser) consumeComment() (string, bool) {
 }
 
 func (p *addrParser) decodeRFC2047Word(s string) (word string, isEncoded bool, err error) {
-	if p.dec != nil {
-		word, err = p.dec.Decode(s)
-	} else {
-		word, err = rfc2047Decoder.Decode(s)
+	dec := p.dec
+	if dec == nil {
+		dec = &rfc2047Decoder
 	}
 
+	// Substitute our own CharsetReader function so that we can tell
+	// whether an error from the Decode method was due to the
+	// CharsetReader (meaning the charset is invalid).
+	// We used to look for the charsetError type in the error result,
+	// but that behaves badly with CharsetReaders other than the
+	// one in rfc2047Decoder.
+	adec := *dec
+	charsetReaderError := false
+	adec.CharsetReader = func(charset string, input io.Reader) (io.Reader, error) {
+		if dec.CharsetReader == nil {
+			charsetReaderError = true
+			return nil, charsetError(charset)
+		}
+		r, err := dec.CharsetReader(charset, input)
+		if err != nil {
+			charsetReaderError = true
+		}
+		return r, err
+	}
+	word, err = adec.Decode(s)
 	if err == nil {
 		return word, true, nil
 	}
 
-	if _, ok := err.(charsetError); ok {
+	// If the error came from the character set reader
+	// (meaning the character set itself is invalid
+	// but the decoding worked fine until then),
+	// return the original text and the error,
+	// with isEncoded=true.
+	if charsetReaderError {
 		return s, true, err
 	}
 
diff --git a/src/net/mail/message_test.go b/src/net/mail/message_test.go
index 80a17b2853..41c54b8687 100644
--- a/src/net/mail/message_test.go
+++ b/src/net/mail/message_test.go
@@ -344,6 +344,17 @@ func TestAddressParsingError(t *testing.T) {
 			t.Errorf(`mail.ParseAddress(%q) #%d want %q, got %v`, tc.text, i, tc.wantErrText, err)
 		}
 	}
+
+	t.Run("CustomWordDecoder", func(t *testing.T) {
+		p := &AddressParser{WordDecoder: &mime.WordDecoder{}}
+		for i, tc := range mustErrTestCases {
+			_, err := p.Parse(tc.text)
+			if err == nil || !strings.Contains(err.Error(), tc.wantErrText) {
+				t.Errorf(`p.Parse(%q) #%d want %q, got %v`, tc.text, i, tc.wantErrText, err)
+			}
+		}
+	})
+
 }
 
 func TestAddressParsing(t *testing.T) {