Uploading images prompts undefined file size error

Hi guys,
Some users are unable to upload some of their images and they are continuously complaining. They receive the following error message:
Not all users are reporting this problem, just some of them. The issue is that the error message says the size of the current image is "undefined". I don’t know the meaning of this event.
Error:
Message from webpage
2011_06_header_wmoa.jpg cannot be uploaded!
File size (undefined B) is too large. The maximum file size allowed is set to:150 KB.
Please help me with suggestions. Thanks.
