Export-SPWeb invalidData

I have a SharePoint site collection at https://myurl.com/bi and under that a site called help. I’m trying to export a single list (a task list called testtasks) out of there through PowerShell using the following command:

Export-SPWeb –Identity https://myurl.com/bi -path “c:\export\export.cmp” –itemurl “/bi/help/lists/testtasks” –verbose

However, this just fails with the following:

Export-SPWeb: At line:1 char:1
CategoryInfo: InvalidData
FullyQualifiedError: Microsoft.Sharepoint.PowerSHell.SPCmdLetExportWeb

I’m guessing this is some sort of syntax error and reading around it seems to be the –ItemUrl param that’s problematic. However, I’ve tried lots of combinations, including

–itemurl “/bi/help/lists/testtasks”
–itemurl “/help/lists/testtasks”
–itemurl “help/lists/testtasks”
–itemurl “bi/help/lists/testtasks”
–itemurl http://ift.tt/1CAxSZu

But get the same error (or an error telling me its an invalid URL.

I've previously run Add-SPShellAdmin for my user and also given myself db_owner permissions on the DB.

Can anyone fathom what I’m doing wrong here or if I’ve missed something?

Thanks