Shift+Meta4 modifier combination not working

[prg318]

Key bindings that use both Mod4 and Shift do not seem to be properly handled. Key binding directives like this do not seem to work under Arch Linux:

<Key mask="4S" key="#">send#</Key>
<Key mask="4S" key="Return">exec: urxvt</Key>

The same key bindings work with the "Shift" key replaced with "Alt":

<Key mask="4A" key="#">send#</Key>
<Key mask="4A" key="Return">exec: urxvt</Key>

This behavior happens on both the latest release (2.3.7) and with the latest code from git.