Calling ASN1.oidFromSignatureAlgorithm(String) in org.wildfly.security.x500.cert.X509CertificateBuilder.build() method on line 375 [1] for unknown algorithms returns null which is assigned to signatureAlgorithmOid and results to NPE thrown from derEncoder.encodeObjectIdentifier(signatureAlgorithmOid) on line 377.

[1] https://github.com/wildfly-security/wildfly-elytron/blob/e01a09572b02f33db01695eb85638e88b0cd944f/src/main/java/org/wildfly/security/x500/cert/X509CertificateBuilder.java#L375