How to reproduce:
1. Add any GameObject that has more than 1 material to any scene
2. Select the GameObject and in the Mesh Renderer component reduce Material array size (to 1 for example)
2. Open Window/Lighting/Settings
3. Under Mixed Lighting enable Baked Global Illumination
4. Under Lightmapping Settings set Lightmapper to Progressive
5. Press Generate Lighting

Reproducible with: 2018.1.0b9, 2018.1.0f1
Not reproducible with: 5.6.6f1, 2017.1.3p3, 2017.2.2p3, 2017.4.1f1, 2018.1.0b8, 2018.2.0b1
Could not test with: 5.6.0b11, 2017.1.0a3 (Progressive Lightmapper not implemented)

Note that it's also reproducible when Material array size is set to 0 with: 5.6.1p4, 5.6.6f1, 2017.1.0b9, 2017.1.3p3, 2017.2.0a2, 2017.2.2p3, 2017.4.1f1, 2018.1.0f1