OS X crash with Play mode

While building a text-only adventure game, I've been using Play mode for testing. My combat subsystem, a tunnel named `fight` crashes when it's invoked, for some reason. Here's the stack trace:

```
Unhandled Exception:
System.Exception: Shouldn't ever have a null value where it couldn't be found at all
at Ink.Runtime.VariablesState.GetRawVariableWithName (System.String name, Int32 contextIndex) <0x13c6a40 + 0x000af> in :0
at Ink.Runtime.VariablesState.GetVariableWithName (System.String name, Int32 contextIndex) <0x13c69a8 + 0x00023> in :0
at Ink.Runtime.VariablesState.GetVariableWithName (System.String name) <0x13c6978 + 0x0001f> in :0
at Ink.Runtime.Story.PerformLogicAndFlowControl (Ink.Runtime.Object contentObj) <0x13bb608 + 0x0120b> in :0
at Ink.Runtime.Story.Step () <0x13baa20 + 0x00167> in :0
at Ink.Runtime.Story.ContinueInternal () <0x13ba250 + 0x00083> in :0
at Ink.Runtime.Story.Continue () <0x13c6398 + 0x00027> in :0
at Ink.CommandLinePlayer.EvaluateStory () <0x13c6238 + 0x0003f> in :0
at Ink.CommandLinePlayer.Begin () <0x13c5c08 + 0x00567> in :0
at Ink.CommandLineTool..ctor (System.String[] args) <0x128b1c0 + 0x0080f> in :0
at Ink.CommandLineTool.Main (System.String[] args) <0x128af30 + 0x00023> in :0
[ERROR] FATAL UNHANDLED EXCEPTION: System.Exception: Shouldn't ever have a null value where it couldn't be found at all
at Ink.Runtime.VariablesState.GetRawVariableWithName (System.String name, Int32 contextIndex) <0x13c6a40 + 0x000af> in :0
at Ink.Runtime.VariablesState.GetVariableWithName (System.String name, Int32 contextIndex) <0x13c69a8 + 0x00023> in :0
at Ink.Runtime.VariablesState.GetVariableWithName (System.String name) <0x13c6978 + 0x0001f> in :0
at Ink.Runtime.Story.PerformLogicAndFlowControl (Ink.Runtime.Object contentObj) <0x13bb608 + 0x0120b> in :0
at Ink.Runtime.Story.Step () <0x13baa20 + 0x00167> in :0
at Ink.Runtime.Story.ContinueInternal () <0x13ba250 + 0x00083> in :0
at Ink.Runtime.Story.Continue () <0x13c6398 + 0x00027> in :0
at Ink.CommandLinePlayer.EvaluateStory () <0x13c6238 + 0x0003f> in :0
at Ink.CommandLinePlayer.Begin () <0x13c5c08 + 0x00567> in :0
at Ink.CommandLineTool..ctor (System.String[] args) <0x128b1c0 + 0x0080f> in :0
at Ink.CommandLineTool.Main (System.String[] args) <0x128af30 + 0x00023> in :0
```