diff --git a/src/codemodder/dependency_management/pyproject_writer.py b/src/codemodder/dependency_management/pyproject_writer.py
index 6f6c4346..e869fe28 100644
--- a/src/codemodder/dependency_management/pyproject_writer.py
+++ b/src/codemodder/dependency_management/pyproject_writer.py
@@ -13,16 +13,20 @@ def add_to_file(
     ) -> Optional[ChangeSet]:
         pyproject = self._parse_file()
         original = deepcopy(pyproject)
-        pyproject["project"]["dependencies"].extend(
-            [f"{dep.requirement}" for dep in dependencies]
-        )
+
+        try:
+            pyproject["project"]["dependencies"].extend(
+                [f"{dep.requirement}" for dep in dependencies]
+            )
+        except tomlkit.exceptions.NonExistentKey:
+            return None
 
         diff, added_line_nums = create_diff_and_linenums(
             tomlkit.dumps(original).split("\n"), tomlkit.dumps(pyproject).split("\n")
         )
 
         if not dry_run:
-            with open(self.path, "w") as f:
+            with open(self.path, "w", encoding="utf-8") as f:
                 tomlkit.dump(pyproject, f)
 
         changes = [
@@ -47,5 +51,5 @@ def add_to_file(
         )
 
     def _parse_file(self):
-        with open(self.path) as f:
+        with open(self.path, encoding="utf-8") as f:
             return tomlkit.load(f)
diff --git a/tests/dependency_management/test_pyproject_writer.py b/tests/dependency_management/test_pyproject_writer.py
index 1a60d495..fa8fe5b3 100644
--- a/tests/dependency_management/test_pyproject_writer.py
+++ b/tests/dependency_management/test_pyproject_writer.py
@@ -1,7 +1,6 @@
 from textwrap import dedent
 import pytest
 
-from packaging.requirements import Requirement
 from codemodder.dependency_management.pyproject_writer import PyprojectWriter
 from codemodder.dependency import DefusedXML, Security
 from codemodder.project_analysis.file_parsers.package_store import PackageStore
@@ -125,7 +124,7 @@ def test_add_same_dependency_only_once(tmpdir):
 
     writer = PyprojectWriter(store, tmpdir)
     dependencies = [Security, Security]
-    changeset = writer.write(dependencies)
+    writer.write(dependencies)
 
     updated_pyproject = """\
         [build-system]
@@ -178,6 +177,33 @@ def test_dont_add_existing_dependency(tmpdir):
 
     writer = PyprojectWriter(store, tmpdir)
     dependencies = [Security]
-    changeset = writer.write(dependencies)
+    writer.write(dependencies)
+
+    assert pyproject_toml.read() == dedent(orig_pyproject)
+
+
+def test_pyproject_no_dependencies(tmpdir):
+    orig_pyproject = """\
+        [build-system]
+        requires = ["setuptools", "setuptools_scm>=8"]
+        build-backend = "setuptools.build_meta"
+        [project]
+        name = "codemodder"
+    """
+
+    pyproject_toml = tmpdir.join("pyproject.toml")
+    pyproject_toml.write(dedent(orig_pyproject))
+
+    store = PackageStore(
+        type="requirements.txt",
+        file=str(pyproject_toml),
+        dependencies=set(),
+        py_versions=[">=3.10.0"],
+    )
+
+    writer = PyprojectWriter(store, tmpdir)
+    dependencies = [Security]
+
+    writer.write(dependencies)
 
     assert pyproject_toml.read() == dedent(orig_pyproject)