Mac App Store Rejection - App sandbox not enabled

Question

I've submitted my app to the Mac App Store and it validated fine. However, I keep getting Invalid Binary messages with the following;

App sandbox not enabled - The following executables must include the "com.apple.security.app-sandbox" entitlement with a Boolean value of true in the entitlements property list. Refer to the App Sandbox page for more information on sandboxing your app.

This is despite having the entitlements enabled in the app (both the app and the helper) with the following contents;

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>com.apple.security.app-sandbox</key>
    <true/>
</dict>
</plist>

Not sure what to check next.

Answer 1

Okay, solved this one myself and posting here for completeness. It would seem that despite the existence of the entitlement file in the project, unless the app is codesigned it will not recognise the sand box entitlement. This is even though the app validates successfully.