Value of Mesos attributes are supposed to be optional but are not

The Mesos documentation states that "Attributes are key-value pairs (where value is optional optional)" but if you try to provide an agent with an attribute without a value it errors out (output below provided by James Peach):

$ sudo /opt/mesos/agent "--attributes=key1:value1;key2:;key3:value3"
...
F0714 06:16:49.332021 16167 attributes.cpp:145] Invalid attribute key:value pair 'key2:'

$ sudo /opt/mesos/agent "--attributes=key1:value1;key2;key3:value3"
...
F0714 06:17:58.916723 16297 attributes.cpp:145] Invalid attribute key:value pair 'key2'

Either the documentation should be fix or an agent should accept attributes without a value.