Run - time error '1004': Unable to get the Sum property of WorksheetFunction class

Whenever I use backspace in textbox1 to delete the 0 and key in a number, run'time error '1004' appears.
Also, if I run a debug, the one in Bold and color blue is highlighted.

Please help.
Thanks in advance!



Here's the partial code:

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Try testing textbox for numeric content

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You should probably test all textboxes before using their values.

Try this.

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