Android APK signing configurations need to be assigned to the build type too

Today I needed to generate a signed APK to test an Android app in the release configuration. Setting the build variant to "release" and trying to run produced an error about needing to sign the APK. So I created a signing configuration on the Signing tab of the Project Structure dialog (where the error message's Fix button took me), but the problem persisted. Apparently the signing configuration also has to be assigned to the build type; this is not done automatically. The build variants listed on the Built Types tab have a Signing Config field - setting that to the newly created Signing entry does the job.