# What does a “null” result mean in the Census ACS API?

There are ACS 5 variables that are available at the tract level, but return null at the block group level. For example, median monthly housing costs:

http://api.census.gov/data/2014/acs5/?get=B25105_001E&for=tract:*&in=state:17%20county:001

http://api.census.gov/data/2014/acs5/?get=B25105_001E&for=block%20group:*&in=state:17%20county:001%20tract:00100

What does a null value signify? Is it that the variable is never available at the block group level, or just for some/most of the block groups?