ruby on rails local-variable declaration in condition

Question

i hope someone can help me (us). We`ve got a simple embedded ruby file => "index.html.erb" In this file are 5 lines of code.

<% if false %>
    <% mylovlyvar = 0  %>
<% else %>
    <%= mylovlyvar %>
<% end %>

The example above works fine.
But why ????

If i comment the declaration of tthe local var in the if condition like this...

<% if false %>
    <%# mylovlyvar = 0  %>
<% else %>
    <%= mylovlyvar %>
<% end %>

.... i get the error

undefined local variable or method `mylovlyvar' for #<#<Class:0x007f235e573090>:0x007f23606cbcb0>

Can someone please explain what happens in this condition with the local variable declaration.

Thank You :)