The method peopleperSpaceFloorAreapeoplePerSpaceFloorArea returns an optional double (see PeopleDefinition Class Reference). Check out the section "Optionals and .get" from the Measure Writing Guide, and @David Goldwasser's answer.

*Edit to correct method name

The I believe this is because the method peoplePerSpaceFloorAreapeoplePerFloorArea returns an optional double (see PeopleDefinition Class Referencehere). Check out the section "Optionals and .get" from the Measure Writing Guide, and @David Goldwasser's answer.

I believe this is because the method peoplePerFloorArea returns an optional double (see here). Check out the section chapter "Optionals and .get" from the Measure Writing Guide.