Restrict open-end response to letters

Following the solution at:

I inserted this code into the Custom Javascript Validation tab of an open-end question ('suburb')

[js]

if (!/^[a-z]*$/i.test(SSI_GetValue('[% QuestionName() %]'))) {

strErrorMessage = 'Error';

}

[/js]

However, the effect is to stop the NEXT button functioning, regardless of whether letters or numbers are entered in the response.

Any advice on the problem?

thanks

Dan