Fitting parameter when using function arguments

I am using the following function to fit data set, function works well and responds well, but when I try to fit the data using curve fitting I get an error. I am sure it is related to *arg, because when I do not use *arg in the function and write A, B, C, D directly in the function, both function and fitting work well.

Here is the code:

bin=[4,4.5,5,5.5]
arg=['A','B','C','D']

def interpo1(x,*arg):
  result=[] 
  for i in range(len(x)): 
    x2, x1=x[i,0:2]
    if (x1 >= bin[0]) & (x1 <bin[1]):
        if (x2>= bin[0]) & (x2<bin[1]):
            f1=arg[0] + ((x1 -bin[0])/(bin[1]-bin[0]))*(arg[1]-arg[0])
            f2=arg[0] + ((x2 -bin[0])/(bin[1]-bin[0]))*(arg[1]-arg[0])
            kh=f2/f1
        if x2>= bin[1] and x2<bin[2]:
            f1=arg[0] + ((x1 -bin[0])/(bin[1]-bin[0]))*(arg[1]-arg[0])
            f2=arg[1] + ((x2 -bin[1])/(bin[2]-bin[1]))*(arg[2]-arg[1])
            kh=f2/f1
            
        if x2>= bin[2] and x2<bin[3]:
            f1=arg[0] + ((x1 -bin[0])/(bin[1]-bin[0]))*(arg[1]-arg[0])
            f2=arg[2] + ((x2 -bin[2])/(bin[3]-bin[2]))*(arg[3]-arg[2])
            kh=f2/f1
    if x1>= bin[1] and x1<bin[2]:
        if x2>= bin[1] and x2<bin[2]:
            f1=arg[1] + ((x1 -bin[1])/(bin[2]-bin[1]))*(arg[2]-arg[1])
            f2=arg[1] + ((x2 -bin[1])/(bin[2]-bin[1]))*(arg[2]-arg[1])
            kh=f2/f1
        if x2>= bin[2] and x2<bin[3]:
            f1=arg[1] + ((x1 -bin[1])/(bin[2]-bin[1]))*(arg[2]-arg[1])
            f2=arg[2] + ((x2 -bin[2])/(bin[3]-bin[2]))*(arg[3]-arg[2])
            kh=f2/f1
    if x1>= bin[2] and x1<bin[3]:
        if x2>= bin[2] and x2<bin[3]:
            f1=arg[2] + ((x1 -bin[2])/(bin[3]-bin[2]))*(arg[3]-arg[2])
            f2=arg[2] + ((x2 -bin[2])/(bin[3]-bin[2]))*(arg[3]-arg[2])
            kh=f2/f1
    result.append(kh)      
  return result




popt1, pcov1 = curve_fit(interpo1,x, y, method='lm')

A,B,C,D=popt1

And here is the error that I get:

    popt1, pcov1 = curve_fit(interpo1,x, y, method='lm')

  File /opt/anaconda3/lib/python3.9/site-packages/scipy/optimize/minpack.py:710 in curve_fit
    raise ValueError("Unable to determine number of fit parameters.")

ValueError: Unable to determine number of fit parameters.

Many thanks in advance for your help!

Following this question I added the following part, but, again, I get the same error.

def wrapper_fit_func(x, *arg):
    A, B, C,D = list(arg[0]), list(arg[1]), list(arg[2]), list(arg[3]) 
    return interpo1(x, A, B, C,D)

popt1, pcov1 = curve_fit(wrapper_fit_func,x, y, method='lm')