diff --git a/.github/actions/build-project/action.yml b/.github/actions/build-project/action.yml
index fe8b7a0..76e0be3 100644
--- a/.github/actions/build-project/action.yml
+++ b/.github/actions/build-project/action.yml
@@ -14,6 +14,17 @@ runs:
       with:
         python-version: ${{ inputs.python-version }}
 
+    - name: Install lxml system deps (Linux only)
+      shell: bash
+      if: runner.os == 'Linux'
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y \
+          libxml2-dev \
+          libxslt1-dev \
+          python3-dev \
+          build-essential
+
     - name: Install host python dependencies
       shell: bash
       run: |
diff --git a/.github/workflows/cd-master.yml b/.github/workflows/cd-master.yml
index 9040e65..a4eb015 100644
--- a/.github/workflows/cd-master.yml
+++ b/.github/workflows/cd-master.yml
@@ -24,7 +24,7 @@ jobs:
       - name: Build the project
         uses: ./.github/actions/build-project
         with:
-          python-version: '3.x'
+          python-version: '3.9'
 
   gh-pages:
     needs: build
@@ -35,7 +35,7 @@ jobs:
         uses: actions/download-artifact@v4
         with:
           path: ./build
-          pattern: build.*.Linux-py3.x
+          pattern: build.*.Linux-py3.9
           merge-multiple: true
 
       - name: Deploy to Github Pages
diff --git a/.github/workflows/ci-develop.yml b/.github/workflows/ci-develop.yml
index 66d075f..b602289 100644
--- a/.github/workflows/ci-develop.yml
+++ b/.github/workflows/ci-develop.yml
@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ['3.x', '3.9']
+        python-version: ['3.9', '3.10', '3.11', '3.12', '3.13']
         os: [ubuntu-latest, windows-latest]
 
     name: Build ${{ matrix.os }}-py${{ matrix.python-version }}
diff --git a/.github/workflows/pr-checks.yml b/.github/workflows/pr-checks.yml
index a2b3f18..be99fcb 100644
--- a/.github/workflows/pr-checks.yml
+++ b/.github/workflows/pr-checks.yml
@@ -36,7 +36,7 @@ jobs:
       - name: Set up python
         uses: actions/setup-python@v5
         with:
-          python-version: '3.x'
+          python-version: '3.9'
 
       - name: Lint sources
         shell: bash
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 46e23d0..504fad2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,11 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [4.0.2] - 2025-12-22
+
+### Fixed
+ - API credentials checking from [Issue #42](https://github.com/b3yc0d3/rule34Py/issues/42)
+
 ## [4.0.1] - 2025-08-31
 
 ### Changed
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644
index 0000000..4d19ee2
--- /dev/null
+++ b/CONTRIBUTING.md
@@ -0,0 +1,36 @@
+# Contributing
+
+Thanks for taking an interest in contributing to the rule34Py project!
+
+* This project's **canonical upstream** is at https://github.com/b3yc0d3/rule34Py.
+* File **bugs**, **enhancement requests**, and other **issues** to the GH issue tracker at https://github.com/b3yc0d3/rule34Py/issues.
+* See the [Developer Guide](https://b3yc0d3.github.io/rule34Py/dev/developer-guide.html) for information about how to **build** this project from source and run tests.
+
+
+## Submitting Changes
+
+#. Base your development branch off of the [upstream](https://github.com/b3yc0d3/rule34Py/tree/develop) ``develop`` reference.
+
+#. Before committing your changes, run the **project linter** using ``make``. It will use the ``ruff`` to lint all the project sources.
+
+    .. code-block:: bash
+
+        poetry install  # Optional, if you have not done it previously.
+        make lint
+
+    Fix or respond to any findings in the linter.
+
+#. Run the project's test suite against your changes. Ensure that all tests pass.
+
+    .. code-block:: bash
+
+        poetry install  # Optional, if you have not done it previously.
+        make check
+
+#. Write a good commit message. If you are unsure of how, [this cbeams article](https://cbea.ms/git-commit/) gives reasonable suggestions. Commit your changes.
+
+#. Fork the canonical upstream repository on github. \[[GitHub Docs](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/fork-a-repo)\]
+
+#. Push your development branch to your own fork. [Open a new Pull Request](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request-from-a-fork) against the upstream `develop` ref.
+
+#. Submit your PR. Respond to any PR build failures or feedback from the maintainers.
diff --git a/pyproject.toml b/pyproject.toml
index 35d3117..3c1fc99 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -30,7 +30,7 @@ maintainers = [
 ]
 readme = "README.md"
 requires-python = ">=3.9, <4.0"
-version = "4.0.1"
+version = "4.0.2"
 
 
 [project.urls]
diff --git a/rule34Py/__init__.py b/rule34Py/__init__.py
index 6831322..bb38143 100644
--- a/rule34Py/__init__.py
+++ b/rule34Py/__init__.py
@@ -1,6 +1,6 @@
 # rule34Py - Python api wrapper for rule34.xxx
 #
-# Copyright (C) 2022-2024 b3yc0d3 <b3yc0d3@gmail.com>
+# Copyright (C) 2022-2025 b3yc0d3 <b3yc0d3@gmail.com>
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
diff --git a/rule34Py/rule34.py b/rule34Py/rule34.py
index 14ae0cb..f3d797b 100644
--- a/rule34Py/rule34.py
+++ b/rule34Py/rule34.py
@@ -138,7 +138,7 @@ def _get(self, *args, **kwargs) -> requests.Response:
         is_api_request = args[0].startswith(__api_url__) == True
 
         # check if api credentials are set
-        if is_api_request and self.user_id == None and self.api_key == None:
+        if is_api_request and self.user_id == None or self.api_key == None or (self.user_id == None and self.api_key == None):
             raise ValueError(
                 "API credentials must be supplied, api_key and user_id can not be None!\nSee https://api.rule34.xxx/ for more information."
             )
diff --git a/tests/unit/test_rule34Py.py b/tests/unit/test_rule34Py.py
index 22fb3eb..fdaa68c 100644
--- a/tests/unit/test_rule34Py.py
+++ b/tests/unit/test_rule34Py.py
@@ -250,3 +250,25 @@ def test_rule34Py_top_tags(rule34):
     assert isinstance(top_tags, list)
     assert len(top_tags) == 100
     assert isinstance(top_tags[0], TopTag)
+
+def test_rule34Py_api_key(rule34):
+    rule34.api_key = None
+    with pytest.raises(ValueError) as execinfo:
+        rule34.top_tags()
+        
+    assert str(execinfo.value) == "API credentials must be supplied, api_key and user_id can not be None!\nSee https://api.rule34.xxx/ for more information."
+
+def test_rule34Py_user_id(rule34):
+    rule34.user_id = None
+    with pytest.raises(ValueError) as execinfo:
+        rule34.top_tags()
+        
+    assert str(execinfo.value) == "API credentials must be supplied, api_key and user_id can not be None!\nSee https://api.rule34.xxx/ for more information."
+
+def test_rule34Py_credentials(rule34):
+    rule34.api_key = None
+    rule34.user_id = None
+    with pytest.raises(ValueError) as execinfo:
+        rule34.top_tags()
+        
+    assert str(execinfo.value) == "API credentials must be supplied, api_key and user_id can not be None!\nSee https://api.rule34.xxx/ for more information."
\ No newline at end of file