pkg productsign. What am I doing wrong?

Question

Apple Developer Program

> spctl --assess -v --type install Unsigned.pkg
Unsigned.pkg: rejected
source=no usable signature

> productsign --timestamp=none --sign "Developer ID Installer: Roga and Kopyta Ltd." Unsigned.pkg Signed.pkg
productsign: signing product with identity "Developer ID Installer: Roga and Kopyta Ltd. (123456789)" from keychain /Users/user/Library/Keychains/login.keychain
productsign: adding certificate "Developer ID Certification Authority"
productsign: adding certificate "Apple Root CA"
productsign: Wrote signed product archive to Signed.pkg

> spctl --assess -v  Signed.pkg
Signed.pkg: rejected
source=no usable signature

> spctl --assess -v --type install Signed.pkg
Signed.pkg: accepted
source=Developer ID

We launch and...

Signed.pkg can't be installed because its digital signature is invalid.