diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 11a5c88..c90fd99 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -42,7 +42,7 @@ jobs:
       fail-fast: false
       matrix:
         python-version: ["3.13"]
-        tox-job: ["linters", "twine-check"]
+        tox-job: ["linters", "mypy", "twine-check"]
 
     steps:
     - uses: actions/checkout@v4
diff --git a/duplicate_url_discarder_rules/__init__.py b/duplicate_url_discarder_rules/__init__.py
index b6bb37b..26ae940 100644
--- a/duplicate_url_discarder_rules/__init__.py
+++ b/duplicate_url_discarder_rules/__init__.py
@@ -16,7 +16,7 @@
 RULE_PATHS_ARTICLE: List[str] = []
 RULE_PATHS_PRODUCT: List[str] = []
 
-for path in RULE_PATHS:
+for path in RULE_PATHS or []:
     filename = Path(path).name
     if filename == "article.json":
         RULE_PATHS_ARTICLE.append(path)
diff --git a/pyproject.toml b/pyproject.toml
index a55df0b..9bc7664 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -40,3 +40,15 @@ target-version = ["py39", "py310", "py311", "py312", "py313"]
 [tool.isort]
 profile = "black"
 multi_line_output = 3
+
+[tool.mypy]
+strict = true
+implicit_reexport = true
+
+[[tool.mypy.overrides]]
+module = "tests.*.*"
+disallow_untyped_defs = false
+
+[[tool.mypy.overrides]]
+module = "url_matcher"
+ignore_missing_imports = true
diff --git a/tests/normalizer/test_normalizer.py b/tests/normalizer/test_normalizer.py
index c520993..5c12f11 100644
--- a/tests/normalizer/test_normalizer.py
+++ b/tests/normalizer/test_normalizer.py
@@ -5,6 +5,7 @@
 
 
 def test_normalizer_main_rules():
+    assert RULE_PATHS is not None
     rule_path = [path for path in RULE_PATHS if path.endswith("normalizer/main.json")]
     assert len(rule_path) == 1
 
diff --git a/tests/queryRemovalExcept/test_query_removal_except.py b/tests/queryRemovalExcept/test_query_removal_except.py
index cfbddf5..f855e36 100644
--- a/tests/queryRemovalExcept/test_query_removal_except.py
+++ b/tests/queryRemovalExcept/test_query_removal_except.py
@@ -5,6 +5,7 @@
 
 
 def test_query_removal_except_product_rules():
+    assert RULE_PATHS is not None
     rule_path = [
         path for path in RULE_PATHS if path.endswith("queryRemovalExcept/product.json")
     ]
diff --git a/tests/subpathRemoval/test_subpath_removal.py b/tests/subpathRemoval/test_subpath_removal.py
index 8284d55..16f4337 100644
--- a/tests/subpathRemoval/test_subpath_removal.py
+++ b/tests/subpathRemoval/test_subpath_removal.py
@@ -5,6 +5,7 @@
 
 
 def test_subpath_removal_product_rules():
+    assert RULE_PATHS is not None
     rule_path = [
         path for path in RULE_PATHS if path.endswith("subpathRemoval/product.json")
     ]
diff --git a/tests/test_rules.py b/tests/test_rules.py
index 3e391d7..b774bfd 100644
--- a/tests/test_rules.py
+++ b/tests/test_rules.py
@@ -11,6 +11,7 @@
 
 
 def test_rule_validity():
+    assert RULE_PATHS is not None
     for path in RULE_PATHS:
         try:
             with open(path, "r") as f:
@@ -45,6 +46,7 @@ def test_rule_validity():
 
 
 def test_rules_concat():
+    assert RULE_PATHS is not None
     all_rules = RULE_PATHS_COMMON + RULE_PATHS_ARTICLE + RULE_PATHS_PRODUCT
     assert isinstance(all_rules, list)
     for path in all_rules:
diff --git a/tox.ini b/tox.ini
index 217c2db..a28e146 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = py39,py310,py311,py312,py313,linters,twine-check
+envlist = py39,py310,py311,py312,py313,linters,mypy,twine-check
 
 [testenv]
 deps =
@@ -15,6 +15,12 @@ commands =
 deps = pre-commit
 commands = pre-commit run --all-files --show-diff-on-failure
 
+[testenv:mypy]
+deps =
+    mypy==1.13.0
+    duplicate-url-discarder
+commands = mypy duplicate_url_discarder_rules tests
+
 [testenv:twine-check]
 deps =
     build