Is it safe to assume that you're using a variable as the value for the lease duration? The error says that the variable either doesn't exist or that it contains a $null value.
If you can't find where the problem lies, try adding Set-PSDebug -Strict at the the beginning of your script. If you try to use an undeclared variable you'll get an error like "The variable '$NewVar' cannot be retrieved because it has not been set." along with the line number in the script.
It may simply be a misspelling of the variable name (or a property name).