Can I use variables to define input port dimensions?

When I use "ml.constant" to define 'Port dimensions' (in this case an Inport), I get an error when I run it. Error evaluating parameter 'PortDimensions' in '<path>' Undefined variable "ml" or class "ml.constant".

When I use "constant" to define 'Port dimensions', I get an error when I click on the 'Apply' button. Error:Error invoking object method Dimensions specified in '<path>' are invalid. Dimensions must be -1 (inherited) or a non-empty vector of strictly positive integers.

I know that the value "constant" is a valid workspace variable, and I am able to use it elsewhere.