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Consider the following example:

Manipulate[{1}[[i]], {i, {1 -> "a"}}]

Execute it and you'll get:

enter image description here

The correct result will be displayed if you select a from the popup menu, but the red box won't go away:

enter image description here

The issue doesn't raise if I build something similar in a lower level way:

DynamicModule[{x}, {PopupMenu[Dynamic@x, {1 -> "a"}], Dynamic@{1}[[x]]}]

enter image description here

What am I doing wrong here? Or it's a bug?

OK, it's not a bug. Here's the response from WRI:

Yes, the first implementation (this refers to my first code sample) is an inappropriate way of specifying a control in Manipulate.

Proper ways are shown in Kuba's comment and Chris Degnen's answer below.

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  • $\begingroup$ Use Initialization, I wouldn't say controllers are responsible for initial values and logical content of body of Manipulate. It's user task to make it reasonable. $\endgroup$ – Kuba♦ Oct 11 '15 at 21:53
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Setting the initial value stops the error.

Manipulate[{1, 2}[[i]], {{i, 2}, {1 -> "a", 2 -> "b"}}]
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