diff --git a/rsconnect/environment.py b/rsconnect/environment.py
index 85bd1037..a76f14e2 100644
--- a/rsconnect/environment.py
+++ b/rsconnect/environment.py
@@ -140,7 +140,10 @@ def create_python_environment(
         _warn_on_ignored_manifest(directory)
         _warn_if_environment_directory(directory)
 
-        python_version_requirement = pyproject.detect_python_version_requirement(directory)
+        try:
+            python_version_requirement = pyproject.detect_python_version_requirement(directory)
+        except pyproject.TOMLDecodeError as err:
+            raise RSConnectException(f"Error reading pyproject.toml: {err}") from err
         _warn_on_missing_python_version(python_version_requirement)
 
         _check_requirements_file(directory, requirements_file)
diff --git a/tests/test_environment.py b/tests/test_environment.py
index cbf364b9..f1bef954 100644
--- a/tests/test_environment.py
+++ b/tests/test_environment.py
@@ -293,6 +293,16 @@ def test_is_not_executable(self):
 
 
 class TestPythonVersionRequirements:
+    def test_malformed_pyproject(self, tmp_path):
+        (tmp_path / "pyproject.toml").write_text('[project\nname = "x"\n')
+        (tmp_path / "requirements.txt").write_text("flask\n")
+
+        with pytest.raises(RSConnectException, match="Error reading pyproject.toml:") as exc_info:
+            Environment.create_python_environment(str(tmp_path))
+
+        assert isinstance(exc_info.value.__cause__, rsconnect.environment.pyproject.TOMLDecodeError)
+        assert str(exc_info.value.__cause__) in str(exc_info.value)
+
     def test_pyproject_toml(self):
         env = Environment.create_python_environment(
             os.path.join(TESTDATA, "python-project", "using_pyproject"), requirements_file=None
diff --git a/tests/test_main.py b/tests/test_main.py
index 94a59c2f..65dcf1e9 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -2154,6 +2154,21 @@ def test_no_package_json(self, tmp_path):
 
 
 class TestWriteManifestFramework:
+    def test_malformed_pyproject_with_requirements_file(self, tmp_path):
+        (tmp_path / "app.py").write_text("app = object()\n")
+        (tmp_path / "requirements.txt").write_text("flask\n")
+        (tmp_path / "pyproject.toml").write_text('[project\nname = "x"\n')
+
+        runner = CliRunner()
+        result = runner.invoke(cli, ["write-manifest", "api", str(tmp_path)])
+
+        assert result.exit_code == 1
+        assert "Error: Error reading pyproject.toml:" in result.output
+        assert "line 1" in result.output
+        assert "Traceback" not in result.output
+        assert "Internal error" not in result.output
+        assert not (tmp_path / "manifest.json").exists()
+
     @pytest.mark.parametrize("existing_requirements", [False, True])
     def test_force_generate_requirements(self, tmp_path, existing_requirements):
         (tmp_path / "app.py").write_text("app = object()\n")