How do i access a non static property from another file?

In my case, i'm updatind the line number everytime i get a '\n'. So i have in the .fsl file:

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..
lexbuf.EndPos <- (lexbuf.EndPos:Position).NextLine
...

When parsing the inputed file, a "parse error" is thrown. So in my AnotherFile.fs i have:

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try
	let c = My_parser.main My_lexer.token lb in
	close_in cin;
	c
with e -> 
	close_in cin;
	raise (SyntaxError (Lexing.lexeme_start lb))

But i want to have the line number where the error is. So i tried:

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let a = (Lexing.lexbuf.EndPos:Lexing.Position).NextLine in ...

But that gives me: "Error 1 property 'EndPos' is not static. "

How can i solve this?