What steps will reproduce the problem?
1. Create a macro like this: /mount [nocombat,noflyable][modifier:shift] FastGround pre
2. Use that macro somewhere its conditions are not met (e.g. middle of Orgrimmar, as long as you're Horde)

What is the expected output? What do you see instead?
I expect no action, similar to the behavior of the /cast command.  Instead I get a Lua error:
Message: ..\AddOns\Mountiful\Mountiful.lua line 680:
   bad argument #1 to 'strlower' (string expected, got nil)
Debug:
   [C]: ?
   [C]: strlower()
   Mountiful\Mountiful.lua:680 Mountiful_Command()
   Mountiful\Mountiful.lua:141 value()
   ..\FrameXML\ChatFrame.lua:4147 ChatEdit_ParseText()
   ..\FrameXML\ChatFrame.lua:3737 ChatEdit_SendText()
   ..\FrameXML\ChatFrame.lua:2536
      ..\FrameXML\ChatFrame.lua:2529
   [C]: UseAction()
   ..\FrameXML\SecureTemplates.lua:274 handler()
   ..\FrameXML\SecureTemplates.lua:541
      ..\FrameXML\SecureTemplates.lua:489
   [C]: ?
   ..\FrameXML\SecureHandlers.lua:264
      ..\FrameXML\SecureHandlers.lua:261
   [C]: ?
   ..\FrameXML\SecureHandlers.lua:294
      ..\FrameXML\SecureHandlers.lua:277
   (tail call): ?

What version of the product are you using?
r210

Please provide any additional information below.
I will attach a patch that corrects this while still preserving the behavior of "/mount" alone opening the options window.