I caught this error when I ran the following command.
add key "timer" with value max(0,DATA.Get("timer")-1)
The problem arose as I had previously defined the contents of DATA.get("time") as an int, and I had changed it to text. My mistake. Either way, there's a casting issue and this throws up the following error when executed in the browser. So the error could have arisen because of the subtraction, or the MAX function. This might be useful to other people who are trying to decipher the error. If you see an assertion error, check if you are misusing a text variable. It might be worth to 
Ashley or the developers to put in an error check to help catch the problem or to help a developer diagnose the failure if they've made the same mistake.
Assertion failure: Calling expvalue.set_float without number type
Stack trace:
Error
at assert2 (http://localhost:50000/preview_prelude.js:16:4)
at ExpValue.prototype.set_float (http://localhost:50000/expressions.js:788:3)
at SysExps.prototype.max (http://localhost:50000/system.js:2054:3)
at ExpNode.prototype.eval_system_exp (http://localhost:50000/expressions.js:255:3)
at Parameter.prototype.get_exp (http://localhost:50000/eveng.js:2049:3)
at Action.prototype.run_object (http://localhost:50000/eveng.js:1824:7)
at EventBlock.prototype.run_actions_and_subevents (http://localhost:50000/eveng.js:929:4)
at EventBlock.prototype.run (http://localhost:50000/eveng.js:886:4)
at EventSheet.prototype.run (http://localhost:50000/eveng.js:224:4)
at EventInclude.prototype.run (http://localhost:50000/eveng.js:2318:13)