my app keep showing bad argument to 155.0*empty-string*

i am new in appinventor so i bought the book and try to do one app,

can anyone show me where  i wrong?

puzzleproject.aia

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From just looking at your error message, it looks

like you are trying to subtract two numbers,

but one of them has not been initialized to zero (0),

but instead has been left as a string.

If that string is coming from a text input box,

give it an initial value of 0.

If that input is coming from a global variable,

initialize it to the number 0.

If your input is coming from a user and you have

no control over it, test if it is a number before you

try to do any math on it, and assign the number 0

to it if it isn't a number.

There is a math block for such a test.
P.S.  Screen shots get faster attention than .aia files.

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Use alerts to try to narrow down you problems.

I was able to get the error to stop by changing the "InitPositionImgX" Select list item to2.

by putting a alert in there i was able to see that "InitPositionImgX" had no value when it was used later to subtract!

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Thanks Abraham Getzler tought me the theory ,but i still don't understand where i should change the block. 
Thanks Richard Thompson2 show me how to  narrow down my problems,but when i change  the blocks like you show me ,it still show error like '',2633".

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If you are asking for help, I recommend you to make it as easy for others to be able to help you ...

You probably will get more feedback then...

which means in your case post a screenshot of your relevant blocks...

To download the aia file, upload it to  App Inventor, open it, do some bug hunting for you, etc... this takes time, and most people will not do that...

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