Burning Desire stack - "shell" command problem

[Barry Levine]

I have a field named "theImageToCreate" containing the following text:

hdiutil create -fs HFS+ -volname MyDisk -size 100m 
~/Desktop/MyDiskImage2.dmg

I have a button with the following script:

on mouseUp
   shell (field "theImageToCreate")
end mouseUp

When the script executes, I get an error. Here's the error dialog info:

Type	Handler: can't find handler
Object	Create disk image file
Line	shell (field "theImageToCreate")
Hint	shell


However, when I copy the text in the field and paste it into a Terminal 
window (and then hit "enter"), the command executes and the Terminal 
window even gives me a verbose feedback.

I thought it might be because I had not set the "shellCommand" property 
but that doesn't seem to be the case. I've tried altering the script 
to:

shell (line 1 of field "theImageToCreate")

...but that results in the same error.

Any assistance would be greatly appreciated.

Thanks,
Barry