WiX burn bootstrapping run different InstallCommand parameter for silent install

I have a Wix installer which needs to use a bootstrapper. I have included an extract of the Bundle.wxs below. It shows the Chain, which is to first install .Net 4.5.2 and then depending upon whether the installer has been called with -s for silent install calls ExePackage with or without the InstallCommand with a value of /S. The "OtherInstaller" is a NSIS (nulscript installer) installer so requires a case sensitive /S to trigger it's silent install. I understand that UILevel=2 is the condition to check for for a silent install, but for some reason "OtherInstaller" is not getting called with the /S silent argument. Then after that the "MainMsiInstaller" is called.

<Chain>      
  <PackageGroupRef Id="NetFx452Web"/>      

  <ExePackage Id="OtherInstallerLoud" 
              SourceFile="..\..\bootstrapper\OtherInstallerFile" 
              InstallCondition="NOT UILevel=2"/>

  <ExePackage Id="OtherInstallerSilent"
              SourceFile="..\..\bootstrapper\\OtherInstallerFile"
              InstallCommand="/S "
              InstallCondition="UILevel=2"/>

  <MsiPackage Id="MainMsiInstaller" 
              DisplayInternalUI="yes" 
              SourceFile="..\..\bin\$(var.CandleCfgName)\MainMsiInstaller.msi" />
</Chain>

Any help appreciated.