could not resolve <custom:DialogTitle> to component implementation error in flex

I am getting the following error ...

could not resolve <custom:DialogTitle> to component 开发者_运维问答implementation   

What does this mean and how can I resolve it?

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Be sure to extend a flex component that is close to what you would like to do and @2DH is right.

For example do something like "DialogTitle extends TitleWindow"