diff --git a/stdnum/pl/nip.py b/stdnum/pl/nip.py
index bfc7506e..537356d8 100644
--- a/stdnum/pl/nip.py
+++ b/stdnum/pl/nip.py
@@ -45,10 +45,15 @@ def compact(number: str) -> str:
     return number
 
 
+_weights = (6, 5, 7, 2, 3, 4, 5, 6, 7, -1)
+
+
 def checksum(number: str) -> int:
     """Calculate the checksum."""
-    weights = (6, 5, 7, 2, 3, 4, 5, 6, 7, -1)
-    return sum(w * int(n) for w, n in zip(weights, number)) % 11
+    total = 0
+    for w, n in zip(_weights, number):
+        total += w * (ord(n) - 48)
+    return total % 11
 
 
 def validate(number: str) -> str:
diff --git a/stdnum/pl/pesel.py b/stdnum/pl/pesel.py
index 38ff544e..e7d1fb33 100644
--- a/stdnum/pl/pesel.py
+++ b/stdnum/pl/pesel.py
@@ -88,12 +88,16 @@ def get_gender(number: str) -> str:
         return 'M'
 
 
+_weights = (1, 3, 7, 9, 1, 3, 7, 9, 1, 3)
+
+
 def calc_check_digit(number: str) -> str:
     """Calculate the check digit for organisations. The number passed
     should not have the check digit included."""
-    weights = (1, 3, 7, 9, 1, 3, 7, 9, 1, 3)
-    check = sum(w * int(n) for w, n in zip(weights, number))
-    return str((10 - check) % 10)
+    total = 0
+    for w, n in zip(_weights, number):
+        total += w * (ord(n) - 48)
+    return str((10 - total) % 10)
 
 
 def validate(number: str) -> str: