diff --git a/decode.go b/decode.go
index a04981f..04e0eff 100644
--- a/decode.go
+++ b/decode.go
@@ -2,8 +2,10 @@ package logfmt
 
 import (
 	"bufio"
+	"bytes"
 	"fmt"
 	"io"
+	"unicode/utf8"
 )
 
 // A Decoder reads and decodes logfmt records from an input stream.
@@ -68,13 +70,19 @@ func (dec *Decoder) ScanKeyval() bool {
 	return false
 
 key:
-	start := dec.pos
+	const invalidKeyError = "invalid key"
+
+	start, multibyte := dec.pos, false
 	for p, c := range line[dec.pos:] {
 		switch {
 		case c == '=':
 			dec.pos += p
 			if dec.pos > start {
 				dec.key = line[start:dec.pos]
+				if multibyte && bytes.IndexRune(dec.key, utf8.RuneError) != -1 {
+					dec.syntaxError(invalidKeyError)
+					return false
+				}
 			}
 			if dec.key == nil {
 				dec.unexpectedByte(c)
@@ -89,13 +97,23 @@ key:
 			dec.pos += p
 			if dec.pos > start {
 				dec.key = line[start:dec.pos]
+				if multibyte && bytes.IndexRune(dec.key, utf8.RuneError) != -1 {
+					dec.syntaxError(invalidKeyError)
+					return false
+				}
 			}
 			return true
+		case c >= utf8.RuneSelf:
+			multibyte = true
 		}
 	}
 	dec.pos = len(line)
 	if dec.pos > start {
 		dec.key = line[start:dec.pos]
+		if multibyte && bytes.IndexRune(dec.key, utf8.RuneError) != -1 {
+			dec.syntaxError(invalidKeyError)
+			return false
+		}
 	}
 	return true
 
@@ -186,9 +204,6 @@ func (dec *Decoder) Value() []byte {
 	return dec.value
 }
 
-// func (dec *Decoder) DecodeValue() ([]byte, error) {
-// }
-
 // Err returns the first non-EOF error that was encountered by the Scanner.
 func (dec *Decoder) Err() error {
 	return dec.err
diff --git a/decode_test.go b/decode_test.go
index d261880..363152d 100644
--- a/decode_test.go
+++ b/decode_test.go
@@ -118,6 +118,9 @@ func TestDecoder_errors(t *testing.T) {
 		{"a=\"1\\", &SyntaxError{Msg: "unterminated quoted value", Line: 1, Pos: 6}},
 		{"a=\"\\t1", &SyntaxError{Msg: "unterminated quoted value", Line: 1, Pos: 7}},
 		{"a=\"\\u1\"", &SyntaxError{Msg: "invalid quoted value", Line: 1, Pos: 8}},
+		{"a\ufffd=bar", &SyntaxError{Msg: "invalid key", Line: 1, Pos: 5}},
+		{"\x80=bar", &SyntaxError{Msg: "invalid key", Line: 1, Pos: 2}},
+		{"\x80", &SyntaxError{Msg: "invalid key", Line: 1, Pos: 2}},
 	}
 
 	for _, test := range tests {
diff --git a/encode.go b/encode.go
index 4d0fa23..55f1603 100644
--- a/encode.go
+++ b/encode.go
@@ -8,6 +8,7 @@ import (
 	"io"
 	"reflect"
 	"strings"
+	"unicode/utf8"
 )
 
 // MarshalKeyvals returns the logfmt encoding of keyvals, a variadic sequence
@@ -165,11 +166,19 @@ func writeKey(w io.Writer, key interface{}) error {
 }
 
 func invalidKeyRune(r rune) bool {
-	return r <= ' ' || r == '=' || r == '"'
+	return r <= ' ' || r == '=' || r == '"' || r == utf8.RuneError
+}
+
+func invalidKeyString(key string) bool {
+	return len(key) == 0 || strings.IndexFunc(key, invalidKeyRune) != -1
+}
+
+func invalidKey(key []byte) bool {
+	return len(key) == 0 || bytes.IndexFunc(key, invalidKeyRune) != -1
 }
 
 func writeStringKey(w io.Writer, key string) error {
-	if len(key) == 0 || strings.IndexFunc(key, invalidKeyRune) != -1 {
+	if invalidKeyString(key) {
 		return ErrInvalidKey
 	}
 	_, err := io.WriteString(w, key)
@@ -177,7 +186,7 @@ func writeStringKey(w io.Writer, key string) error {
 }
 
 func writeBytesKey(w io.Writer, key []byte) error {
-	if len(key) == 0 || bytes.IndexFunc(key, invalidKeyRune) != -1 {
+	if invalidKey(key) {
 		return ErrInvalidKey
 	}
 	_, err := w.Write(key)
@@ -223,7 +232,7 @@ func writeValue(w io.Writer, value interface{}) error {
 }
 
 func needsQuotedValueRune(r rune) bool {
-	return r <= ' ' || r == '=' || r == '"'
+	return r <= ' ' || r == '=' || r == '"' || r == utf8.RuneError
 }
 
 func writeStringValue(w io.Writer, value string, ok bool) error {
@@ -240,7 +249,7 @@ func writeStringValue(w io.Writer, value string, ok bool) error {
 
 func writeBytesValue(w io.Writer, value []byte) error {
 	var err error
-	if bytes.IndexFunc(value, needsQuotedValueRune) >= 0 {
+	if bytes.IndexFunc(value, needsQuotedValueRune) != -1 {
 		_, err = writeQuotedBytes(w, value)
 	} else {
 		_, err = w.Write(value)
diff --git a/encode_test.go b/encode_test.go
index c7aa1d5..ebebaae 100644
--- a/encode_test.go
+++ b/encode_test.go
@@ -53,6 +53,11 @@ func TestEncodeKeyval(t *testing.T) {
 		{key: decimalStringer{5, 9}, value: "v", want: "5.9=v"},
 		{key: (*decimalStringer)(nil), value: "v", err: logfmt.ErrNilKey},
 		{key: marshalerStringer{5, 9}, value: "v", want: "5.9=v"},
+		{key: "k", value: "\xbd", want: `k="\ufffd"`},
+		{key: "k", value: "\ufffd\x00", want: `k="\ufffd\u0000"`},
+		{key: "k", value: "\ufffd", want: `k="\ufffd"`},
+		{key: "k", value: []byte("\ufffd\x00"), want: `k="\ufffd\u0000"`},
+		{key: "k", value: []byte("\ufffd"), want: `k="\ufffd"`},
 	}
 
 	for _, d := range data {
@@ -82,6 +87,8 @@ func TestMarshalKeyvals(t *testing.T) {
 		{in: kv(), want: nil},
 		{in: kv(nil, "v"), err: logfmt.ErrNilKey},
 		{in: kv(nilPtr, "v"), err: logfmt.ErrNilKey},
+		{in: kv("\ufffd"), err: logfmt.ErrInvalidKey},
+		{in: kv("\xbd"), err: logfmt.ErrInvalidKey},
 		{in: kv("k"), want: []byte("k=null")},
 		{in: kv("k", nil), want: []byte("k=null")},
 		{in: kv("k", ""), want: []byte("k=")},
@@ -125,8 +132,8 @@ func TestMarshalKeyvals(t *testing.T) {
 		if err != d.err {
 			t.Errorf("%#v: got error: %v, want error: %v", d.in, err, d.err)
 		}
-		if got, want := got, d.want; !reflect.DeepEqual(got, want) {
-			t.Errorf("%#v: got '%s', want '%s'", d.in, got, want)
+		if !reflect.DeepEqual(got, d.want) {
+			t.Errorf("%#v: got '%s', want '%s'", d.in, got, d.want)
 		}
 	}
 }
@@ -181,12 +188,12 @@ func (t marshalerStringer) String() string {
 	return fmt.Sprint(t.a + t.b)
 }
 
-var marshalError = errors.New("marshal error")
+var errMarshal = errors.New("marshal error")
 
 type errorMarshaler struct{}
 
 func (errorMarshaler) MarshalText() ([]byte, error) {
-	return nil, marshalError
+	return nil, errMarshal
 }
 
 func BenchmarkEncodeKeyval(b *testing.B) {
diff --git a/fuzz.go b/fuzz.go
index ab916a9..6553b35 100644
--- a/fuzz.go
+++ b/fuzz.go
@@ -12,21 +12,22 @@ import (
 	kr "github.com/kr/logfmt"
 )
 
+// Fuzz checks reserialized data matches
 func Fuzz(data []byte) int {
 	parsed, err := parse(data)
 	if err != nil {
 		return 0
 	}
 	var w1 bytes.Buffer
-	if err := write(parsed, &w1); err != nil {
+	if err = write(parsed, &w1); err != nil {
 		panic(err)
 	}
-	parsed, err = parse(data)
+	parsed, err = parse(w1.Bytes())
 	if err != nil {
 		panic(err)
 	}
 	var w2 bytes.Buffer
-	if err := write(parsed, &w2); err != nil {
+	if err = write(parsed, &w2); err != nil {
 		panic(err)
 	}
 	if !bytes.Equal(w1.Bytes(), w2.Bytes()) {
@@ -35,6 +36,7 @@ func Fuzz(data []byte) int {
 	return 1
 }
 
+// FuzzVsKR checks go-logfmt/logfmt against kr/logfmt
 func FuzzVsKR(data []byte) int {
 	parsed, err := parse(data)
 	parsedKR, errKR := parseKR(data)
@@ -71,7 +73,6 @@ func parse(data []byte) ([][]kv, error) {
 			kvs = append(kvs, kv{dec.Key(), dec.Value()})
 		}
 		got = append(got, kvs)
-		kvs = nil
 	}
 	return got, dec.Err()
 }
diff --git a/jsonstring.go b/jsonstring.go
index 53b6532..030ac85 100644
--- a/jsonstring.go
+++ b/jsonstring.go
@@ -71,7 +71,7 @@ func writeQuotedString(w io.Writer, s string) (int, error) {
 			continue
 		}
 		c, size := utf8.DecodeRuneInString(s[i:])
-		if c == utf8.RuneError && size == 1 {
+		if c == utf8.RuneError {
 			if start < i {
 				buf.WriteString(s[start:i])
 			}
@@ -129,7 +129,7 @@ func writeQuotedBytes(w io.Writer, s []byte) (int, error) {
 			continue
 		}
 		c, size := utf8.DecodeRune(s[i:])
-		if c == utf8.RuneError && size == 1 {
+		if c == utf8.RuneError {
 			if start < i {
 				buf.Write(s[start:i])
 			}
@@ -182,7 +182,7 @@ func unquoteBytes(s []byte) (t []byte, ok bool) {
 			continue
 		}
 		rr, size := utf8.DecodeRune(s[r:])
-		if rr == utf8.RuneError && size == 1 {
+		if rr == utf8.RuneError {
 			break
 		}
 		r += size