diff --git a/src/claude_agent_sdk/_internal/transport/subprocess_cli.py b/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
index 833cba4c..fda5f5bd 100644
--- a/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
+++ b/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
@@ -193,7 +193,7 @@ def _apply_skills_defaults(
 
         Does not mutate the original options object.
         """
-        allowed_tools: list[str] = list(self._options.allowed_tools)
+        allowed_tools: list[str] = list(self._options.allowed_tools or [])
         setting_sources: list[str] | None = (
             list(self._options.setting_sources)
             if self._options.setting_sources is not None
diff --git a/tests/test_transport.py b/tests/test_transport.py
index 1e61e9ad..5a1fdcd8 100644
--- a/tests/test_transport.py
+++ b/tests/test_transport.py
@@ -101,6 +101,31 @@ def test_build_command_strict_mcp_config(self):
         transport = SubprocessCLITransport(prompt="test", options=make_options())
         assert "--strict-mcp-config" not in transport._build_command()
 
+    def test_build_command_allowed_tools_none(self):
+        """Regression: passing ``allowed_tools=None`` must not crash.
+
+        The type annotation declares ``list[str]`` with a default factory of
+        ``list``, but Python's runtime does not enforce TypedDict / dataclass
+        type hints — callers (notably wrapper libraries like
+        ``claude-code-telegram``) sometimes set the attribute to ``None`` after
+        construction to express "no restriction". Previously
+        ``_apply_skills_defaults`` called ``list(options.allowed_tools)``
+        unconditionally and raised ``TypeError: 'NoneType' object is not
+        iterable`` before the CLI even started, surfacing as
+        ``"Unexpected error in Claude SDK"`` for the end user.
+
+        The defensive fix is ``list(... or [])`` — equivalent to an empty
+        allowlist, which the CLI treats as "no ``--allowedTools`` flag".
+        """
+        options = make_options()
+        # Simulate downstream code that assigns None after construction.
+        options.allowed_tools = None  # type: ignore[assignment]
+        transport = SubprocessCLITransport(prompt="test", options=options)
+
+        # Must not raise.
+        cmd = transport._build_command()
+        assert "--allowedTools" not in cmd
+
     def test_cli_path_accepts_pathlib_path(self):
         """Test that cli_path accepts pathlib.Path objects."""
         from pathlib import Path