Lightmass is crashing when using a material that has "Cast Shadow as Masked" set to true in its properties.
Found in 4.18 P3 CL#3676890
Unable to reproduce in 4.16.3 CL# 3561208 and 4.17.2 CL#3658906, this is a regression.
result: lightmass crashes
expected: lighting built successfully
Head over to the existing Questions & Answers thread and let us know what's up.