Change the formula of a target cell in a self-defined worksheet function

HI Guys

I got a problem when I wrote a self-defined worksheet function. The idea is very simple: change the formula of a target cell. A simplified function is like this:

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if I call the function under the following sub, every is fine

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However, if I call the function in a cell
like: PrintFormula("=PROPHETX|QUOTE!'QNGM8[DTN:NYMEX]?Last'")
I got error, the Range("C1").formula is always empty and the error is Application-defined or object-defined error.

Do you guys have any idea about this problem? Thanks