diff --git a/.github/workflows/main.yaml b/.github/workflows/main.yaml
index 0f7438b2..43ad16fa 100644
--- a/.github/workflows/main.yaml
+++ b/.github/workflows/main.yaml
@@ -48,6 +48,10 @@ jobs:
       with:
         submodules: recursive
         persist-credentials: false
+    - name: Install system libraries for lxml
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y libxml2-dev libxslt1-dev
     - name: Use Python ${{ matrix.python-version }}
       uses: actions/setup-python@v6
       with:
@@ -78,6 +82,10 @@ jobs:
         with:
           submodules: recursive
           persist-credentials: false
+      - name: Install system libraries for lxml
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y libxml2-dev libxslt1-dev
       - name: Set up Python
         uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405
         with:
diff --git a/AGENTS.md b/AGENTS.md
index 3ba8d1ab..4ef186d5 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -4,7 +4,8 @@ Read [CONTRIBUTING.rst](CONTRIBUTING.rst) for code style, linting (e.g. `make li
 
 ## Testing
 
-- Prefer **function-based** pytest tests (module-level `def test_...`) over class-based tests (`class Test...`).
+- When adding tests to a file that already uses class-based pytest structure (e.g. `tests/test_jsonld.py`), add them as methods on the appropriate existing class — do not introduce a parallel standalone function-based test. A repo-wide refactor to function-based tests is planned but not yet landed; until then, match each file's existing structure.
+- For brand-new test files, prefer function-based pytest tests (module-level `def test_...`) over class-based tests.
 - Use descriptive test names that reflect behavior (e.g. `test_remote_context_via_link_alternate`).
 
 ## Committing
diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
index 00000000..dc1d0803
--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1 @@
+See [AGENTS.md](AGENTS.md) for project-specific agent guidance.
diff --git a/lib/pyld/jsonld.py b/lib/pyld/jsonld.py
index ce18f0d3..46c6a45e 100644
--- a/lib/pyld/jsonld.py
+++ b/lib/pyld/jsonld.py
@@ -5842,8 +5842,8 @@ def _expand_iri(
 
         # resolve against base
         rval = value
-        # if there is a base in the active context, use that
-        if '@base' in active_ctx:
+        # if a base was passed and the active context has a base, use that
+        if base is not None and '@base' in active_ctx:
             # the None case preserves rval as potentially relative
             if active_ctx['@base'] is not None:
                 resolved_base = (
diff --git a/tests/test_jsonld.py b/tests/test_jsonld.py
index a12016fe..578e3a79 100644
--- a/tests/test_jsonld.py
+++ b/tests/test_jsonld.py
@@ -180,6 +180,36 @@ def test_missing_base(self):
         got = jsonld.expand(input)
         assert got == expected
 
+    def test_base_does_not_expand_property_terms(self):
+        """
+        Regression test: @base must not be used to expand property keys.
+
+        Property names are expanded vocabulary-relative: @vocab, term
+        definitions, compact IRIs with a defined prefix, etc. The active
+        context's @base is for document-relative IRI resolution where the
+        algorithms pass that flag (e.g. certain @id and @type values), not
+        for turning arbitrary keys into absolute IRIs. Here the context sets
+        only @base; `name` has no term definition and no @vocab, so it
+        cannot become an absolute property IRI and must be dropped.
+
+        See: https://www.w3.org/TR/json-ld11-api/#iri-expansion
+        """
+        doc = {
+            '@context': {'@base': 'https://schema.org/'},
+            '@id': 'https://w3.org/yaml-ld/',
+            '@type': 'WebContent',
+            'name': 'YAML-LD',
+        }
+        result = jsonld.expand(doc)
+        # `name` has no vocabulary-relative mapping (@vocab or term
+        # definition); @base must not supply one. The key is dropped.
+        assert result == [
+            {
+                '@id': 'https://w3.org/yaml-ld/',
+                '@type': ['https://schema.org/WebContent'],
+            }
+        ]
+
 
 class TestFrame:
     # Issue 11 - PR: https://github.com/digitalbazaar/pyld/issues/149