I'm trying to create a custom action for my Wix install, and it's just not working, and I'm unsure why.

Here's the bit in the appropriate Wix File:

<Binary Id="INSTALLERHELPER" SourceFile=".\Lib\InstallerHelper.dll" />
<CustomAction Id="HelperAction" BinaryKey="INSTALLERHELPER" DllEntry="CustomAction1" Execute="immediate" />

Here's the full class file for my custom action:

using Microsoft.Deployment.WindowsInstaller;

namespace InstallerHelper
{
  public class CustomActions
  {
    [CustomAction]
    public static ActionResult CustomAction1(Session session)
    {
      session.Log("Begin CustomAction1");

      return ActionResult.Success;
    }
  }
}

The action is run by a button press in the UI (for now):

  <Control Id="Next" Type="PushButton" X="248" Y="243" Width="56" Height="17" Default="yes" Text="!(loc.WixUINext)" >
      <Publish Event="DoAction" Value="HelperAction">1</Publish>
  </Control>

When I run the MSI, I get this error in the log:

MSI (c) (08:5C) [10:08:36:978]: Connected to service for CA interface.
MSI (c) (08:4C) [10:08:37:030]: Note: 1: 1723 2: SQLHelperAction 3: CustomAction1 4: C:\Users\NATHAN~1.TYL\AppData\Local\Temp\MSI684F.tmp 
Error 1723. There is a problem with this Windows Installer package. A DLL required for this install to complete could not be run. Contact your support personnel or package vendor.  Action SQLHelperAction, entry: CustomAction1, library: C:\Users\NATHAN~1.TYL\AppData\Local\Temp\MSI684F.tmp 
MSI (c) (08:4C) [10:08:38:501]: Product: SessionWorks :: Judge Edition -- Error 1723. There is a problem with this Windows Installer package. A DLL required for this install to complete could not be run. Contact your support personnel or package vendor.  Action SQLHelperAction, entry: CustomAction1, library: C:\Users\NATHAN~1.TYL\AppData\Local\Temp\MSI684F.tmp 

Action ended 10:08:38: SQLHelperAction. Return value 3.
DEBUG: Error 2896:  Executing action SQLHelperAction failed.
The installer has encountered an unexpected error installing this package. This may indicate a problem with this package. The error code is 2896. The arguments are: SQLHelperAction, , 

Neither of the two error codes or messages it gives me is enough to tell me what's wrong. Or perhaps I'm just not understanding what they're saying is wrong.

At first I thought it might be because I was using Wix 3.5, so just to be sure I tried using Wix 3.0, but I get the same error.

Any ideas on what I'm doing wrong?