diff --git a/maths/special_numbers/proth_number.py b/maths/special_numbers/proth_number.py
index dcde9cc61..7596b0402 100644
--- a/maths/special_numbers/proth_number.py
+++ b/maths/special_numbers/proth_number.py
@@ -84,7 +84,7 @@ def isprothnumber(number: int) -> bool :
     num -= 1
     n = 0
     while num%2 == 0 :
-        N = N/2
+        num = num/2
         n += 1
     return num < (2**n)