diff --git a/riff/riff.go b/riff/riff.go
index b225193..9b9f71d 100644
--- a/riff/riff.go
+++ b/riff/riff.go
@@ -136,11 +136,10 @@ func (z *Reader) Next() (chunkID FourCC, chunkLen uint32, chunkData io.Reader, e
 		return FourCC{}, 0, nil, z.err
 	}
 	chunkID = FourCC{z.buf[0], z.buf[1], z.buf[2], z.buf[3]}
-	chunkLen = u32(z.buf[4:])
-	z.chunkLen = chunkLen
-	z.padded = chunkLen&1 == 1
+	z.chunkLen = u32(z.buf[4:])
+	z.padded = z.chunkLen&1 == 1
 	z.chunkReader = &chunkReader{z}
-	return chunkID, chunkLen, z.chunkReader, nil
+	return chunkID, z.chunkLen, z.chunkReader, nil
 }
 
 type chunkReader struct {
diff --git a/webp/decode.go b/webp/decode.go
index 4a91f23..259bac6 100644
--- a/webp/decode.go
+++ b/webp/decode.go
@@ -144,6 +144,9 @@ func decode(r io.Reader, configOnly bool) (image.Image, image.Config, error) {
 				}, nil
 			}
 			wantAlpha = true
+
+		default:
+			return nil, image.Config{}, errInvalidFormat
 		}
 	}
 }