Collection was modified after the enumerator was instantiated.

I am using a grid, with udpate panel, ajax controls etc.
So in my code behind everytime the hit my grid page I am clearing the previous session value stored.
I do this:
foreach (string key in Session.Keys) { if (key.Contains("mytem")) Session[key] = null; } But sometimes when i navigate to other pages and come back to this page it errors out saying:
" Collection was modified after the enumerator was instantiated."
I am trying to remove a particular session key.
Any suggestions?
 
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