Can't generate JWKS using 'ES256'

/**
 * Signature algorithm. Could be one of these values :
 * - HS256:    HMAC using SHA-256 hash algorithm (default)
 * - HS384:    HMAC using SHA-384 hash algorithm
 * - HS512:    HMAC using SHA-512 hash algorithm
 * - RS256:    RSASSA using SHA-256 hash algorithm
 * - RS384:    RSASSA using SHA-384 hash algorithm
 * - RS512:    RSASSA using SHA-512 hash algorithm
 * - ES256:    ECDSA using P-256 curve and SHA-256 hash algorithm
 * - ES384:    ECDSA using P-384 curve and SHA-384 hash algorithm
 * - ES512:    ECDSA using P-521 curve and SHA-512 hash algorithm
 * - none:     No digital signature or MAC value included
 */