Actually you can't add an empty custom field to a post/page.
In my opinion this is not fully correct, one should be able to add whatever string he wants into a custom field, even an empty one.
In my case, I was adding custom fields and the client later should have filled in with some texts that would be echoed in the template.
He didn't know the names of the custom fields so I simply had to add them with an empty value. But this is not possibile and I had to fill in some random text just to bypass the error message. Tried with a space also and it threw the same error "Please provide a custom field value."
What do you people think about this?