Difficulty running SLRN with the openbox menu

[Matt Smith]

<item label="SLRN">
  <action name="Execute"><execute>rxvt -e slrn</execute></action>
</item>

This works for all other CLI applications that I use but not for SLRN.
When I type:

rxvt -e slrn

in a terminal window the program launches as intended.
When I click on the menu item the RXVT window opens briefly then
dissappears.

Has anyone seen this kind of behaviour before?
Cheers,
Matt.