Invalid IL code exception when calling method

I'm trying to find out prime numbers in my script, but when I call my function for that, I get a strange error: InvalidProgramException: Invalid IL code in RSA:isPrim (int): IL_0057: ldloc.0 Code:

 def genPrim (start as int, max as int):
     rdom as int
     k = 2
     while true:
         rdom = Random.Range(start, max)
         if self.isPrim(rdom):
             break
     return rdom
     
 def isPrim (zahl as int):
     finished as bool
     if zahl <= 1:
         finished = true
     if not finished:
         while k**2 <= zahl:
             if zahl % k == 0:
                 finished = true
             k += 1           
     return not finished

I checked everything and I just can't find anything wrong :/