Using Apex:Variable with $CurrentPage.parameters in Visualforce

I have a very simple piece of code that I'm trying.

<apex:variable var="startUrl" value="{!$CurrentPage.parameters.startURL}" />
    {!startUrl}

The above code generates an Unknown property 'startUrl' when trying to compile.

<apex:variable var="startUrl" value="{!$CurrentPage.Name}" />
    {!startUrl}

Above code works just fine and the below code works fine.

{!$CurrentPage.parameters.startURL}

Also the first set of code above that fails, will compile fine if I remove the "{!startUrl} line.

I'm not sure why it is failing.