#Value error using custom function in Excel -

i'm getting #value error when try call custom function. supposed bit of math. see wrong here?

i copied 1 off internet:

source:

http://www.engineerexcel.com/linear-interpolation-vba-function-in-excel/

function lininterp(x, xvalues, yvalues)  'x , y values must in ascending order top bottom. 'x must within range of available data.  x1 = application.worksheetfunction.index(xvalues, application.worksheetfunction.match(x, xvalues, 1)) x2 = application.worksheetfunction.index(xvalues, application.worksheetfunction.match(x, xvalues, 1) + 1)  y1 = application.worksheetfunction.index(yvalues, application.worksheetfunction.match(x, xvalues, 1)) y2 = application.worksheetfunction.index(yvalues, application.worksheetfunction.match(x, xvalues, 1) + 1)  lininterp = y1 + (y2–y1) * (x–x1) / (x2–x1) end function 

this simplified version made thinking worksheet function calls may causing error:

function lininterp(x, x1, x2, y1, y2)   lininterp = y1 + (y2–y1) * (x–x1) / (x2–x1) end function 

my test data in unrelated workbook: (all formatted "general")

a1: 633 a2: 634 b1: 14.968 b2: 15.024 c1 (my x): 633.6 

just plugging actual math cell works expected. calling function throws #value error.

my function saved in module in workbook have saved , added excel add-in.

my sampling of formula , data threw error on hyphens not being interpreted 'minus signs'. in fact, come unicode 8211. retyping them, declaring vars variants , removing ...worksheetfunction... fixed problem.

function lininterp(x, xvalues, yvalues)     dim x1 variant, x2 variant, y1 variant, y2 variant      'x , y values must in ascending order top bottom.     'x must within range of available data.      x1 = application.index(xvalues, application.match(x, xvalues, 1))     x2 = application.index(xvalues, application.match(x, xvalues, 1) + 1)      y1 = application.index(yvalues, application.match(x, xvalues, 1))     y2 = application.index(yvalues, application.match(x, xvalues, 1) + 1)      lininterp = y1 + (y2 - y1) * (x - x1) / (x2 - x1) end function 

moral of story: don't trust find on internet.