diff --git a/pfbudget/transform/nullifier.py b/pfbudget/transform/nullifier.py
index 16106a4..ae37a65 100644
--- a/pfbudget/transform/nullifier.py
+++ b/pfbudget/transform/nullifier.py
@@ -1,6 +1,6 @@
 from copy import deepcopy
 import datetime as dt
-from typing import Sequence
+from typing import Iterable, Sequence
 
 from .exceptions import MoreThanOneMatchError
 from .transform import Transformer
@@ -17,14 +17,14 @@ class Nullifier(Transformer):
     def __init__(self, rules=None):
         self.rules = rules if rules else []
 
-    def transform(self, transactions: Sequence[Transaction]) -> Sequence[Transaction]:
+    def transform(self, transactions: Iterable[Transaction]) -> Sequence[Transaction]:
         """transform
 
         Find transactions that nullify each others, e.g. transfers between banks or
         between bank and credit cards.
 
         Args:
-            transactions (Sequence[Transaction]): ordered sequence of transactions
+            transactions (Sequence[Transaction]): sequence of transactions
 
         Raises:
             MoreThanOneMatchError: if there is more than a match for a single transation
@@ -33,7 +33,7 @@ class Nullifier(Transformer):
             Sequence[Transaction]: nullified sequence of transactions
         """
 
-        result = deepcopy(transactions)
+        result = sorted(deepcopy(transactions))
 
         for i, transaction in enumerate(result[:-1]):
             if matches := [t for t in result[i + 1 :] if self._cancels(transaction, t)]:
@@ -47,29 +47,29 @@ class Nullifier(Transformer):
 
         return result
 
-    def transform_inplace(self, transactions: Sequence[Transaction]) -> None:
-        """_summary_
+    def transform_inplace(self, transactions: Iterable[Transaction]) -> None:
+        """transform_inplace
 
         Find transactions that nullify each others, e.g. transfers between banks or
         between bank and credit cards.
 
         Args:
-            transactions (Sequence[Transaction]): ordered sequence of transactions that
-            will be modified inplace
+            transactions (Sequence[Transaction]): sequence of transactions that will be
+            modified inplace
 
         Raises:
             MoreThanOneMatchError: if there is more than a match for a single transation
         """
 
-        for transaction in transactions:
+        for transaction in sorted(transactions):
             if matches := [t for t in transactions if self._cancels(transaction, t)]:
                 if len(matches) > 1:
                     raise MoreThanOneMatchError(f"{transaction} -> {matches}")
 
                 match = matches[0]
 
-                transaction = self._nullify(transaction)
-                match = self._nullify(match)
+                self._nullify(transaction)
+                self._nullify(match)
 
     def _cancels(self, transaction: Transaction, cancel: Transaction):
         return (
@@ -79,6 +79,9 @@ class Nullifier(Transformer):
             and cancel != transaction
             and cancel.bank != transaction.bank
             and cancel.amount == -transaction.amount
+            # even though this class receives uncategorized transactions, they may have
+            # already been nullified before reaching here
+            and not transaction.category
             and (not cancel.category or cancel.category.name != "null")
             and (
                 any(r.matches(transaction) for r in self.rules) if self.rules else True
diff --git a/tests/test_transform.py b/tests/test_transform.py
index dafdd73..f5fb986 100644
--- a/tests/test_transform.py
+++ b/tests/test_transform.py
@@ -1,5 +1,6 @@
 from datetime import date
 from decimal import Decimal
+import pytest
 
 import mocks.categories as mock
 
@@ -12,6 +13,7 @@ from pfbudget.db.model import (
     TransactionTag,
 )
 from pfbudget.transform.categorizer import Categorizer
+from pfbudget.transform.exceptions import MoreThanOneMatchError
 from pfbudget.transform.nullifier import Nullifier
 from pfbudget.transform.tagger import Tagger
 from pfbudget.transform.transform import Transformer
@@ -48,6 +50,36 @@ class TestTransform:
         for t in transactions:
             assert t.category == TransactionCategory("null", CategorySelector.nullifier)
 
+    def test_nullifier_inplace_unordered(self):
+        transactions = [
+            BankTransaction(date(2023, 1, 2), "A2", Decimal("500"), bank="Bank#2"),
+            BankTransaction(date(2023, 1, 2), "B1", Decimal("-500"), bank="Bank#1"),
+            BankTransaction(date(2023, 1, 1), "A1", Decimal("-500"), bank="Bank#1"),
+            BankTransaction(date(2023, 1, 2), "B2", Decimal("500"), bank="Bank#2"),
+        ]
+
+        for t in transactions:
+            assert not t.category
+
+        categorizer: Transformer = Nullifier()
+        with pytest.raises(MoreThanOneMatchError):
+            categorizer.transform_inplace(transactions)
+
+    def test_nullifier_inplace_repeated(self):
+        transactions = [
+            BankTransaction(date(2023, 1, 2), "A1", Decimal("-500"), bank="Bank#1"),
+            BankTransaction(date(2023, 1, 2), "A2", Decimal("500"), bank="Bank#2"),
+            BankTransaction(date(2023, 1, 2), "B1", Decimal("-500"), bank="Bank#1"),
+            BankTransaction(date(2023, 1, 2), "B2", Decimal("500"), bank="Bank#2"),
+        ]
+
+        for t in transactions:
+            assert not t.category
+
+        categorizer: Transformer = Nullifier()
+        with pytest.raises(MoreThanOneMatchError):
+            categorizer.transform_inplace(transactions)
+
     def test_nullifier_with_rules(self):
         transactions = [
             BankTransaction(date(2023, 1, 1), "", Decimal("-500"), bank="Bank#1"),