Getting io.jsonwebtoken.UnsupportedJwtException: Signed Claims JWSs are not supported using parsePlaintextJws

Question

I am getting a weird exception using jjwt when parsing my JWT(JWS).

Here is code of parsing:

JwtParser parser = Jwts.parserBuilder()
        .setSigningKey(publicKey)
        .build();

//exception thrown
Jws<String> parsedToken = parser.parsePlaintextJws(token);
//works
Jws<Claims> parsedToken1 = parser.parseClaimsJws(token);

Here is my code for creating a token:

token = Jwts.builder()
        .setIssuedAt(Date.from(issuedTime.atZone(ZoneId.systemDefault()).toInstant()))
        .setExpiration(Date.from(expirationTime.atZone(ZoneId.systemDefault()).toInstant()))
        .claim("a", "foo")
        .claim("b", "boo")
        .signWith(secretKey, SignatureAlgorithm.RS256)
        .compact();

Exception - io.jsonwebtoken.UnsupportedJwtException: Signed Claims JWSs are not supported.

What's the reason for this exception and how can it be fixed?