Single Key Binding for VSPackage

Hi,

how do we bind a single key like F1, F2 etc.. for a VSPackage. its working fine with a mod1 like Alt+F1.

<KeyBinding guid="guidVSPackage_ABSuiteCmdSet3" id="cmdidMyCommand3"   
      key1="VK_F1" editor="guidVSStd97" />

but its not working if we give as 

<KeyBinding guid="guidVSPackage_ABSuiteCmdSet3" id="cmdidMyCommand3"   
      key1="VK_F1" mod1="ALT" editor="guidVSStd97" />

Please Suggest..