diff --git a/headroom/providers/anthropic.py b/headroom/providers/anthropic.py
index ef488ee3d..b16251621 100644
--- a/headroom/providers/anthropic.py
+++ b/headroom/providers/anthropic.py
@@ -420,13 +420,14 @@ class AnthropicTokenCounter(TokenCounter):
             # str(block) catch-all would produce.
             tokens += count_content_blocks(content, self.count_text)
 
-        # OpenAI format tool calls
-        if "tool_calls" in message:
-            for tool_call in message.get("tool_calls", []):
-                if isinstance(tool_call, dict):
-                    func = tool_call.get("function") or {}
-                    tokens += self.count_text(coerce_countable_text(func.get("name")))
-                    tokens += self.count_text(coerce_countable_text(func.get("arguments")))
+        # OpenAI format tool calls. Guard the value, not just the key: an
+        # OpenAI-format assistant message often carries `tool_calls: null` on a
+        # no-tool turn, and `for ... in None` would raise TypeError.
+        for tool_call in message.get("tool_calls") or []:
+            if isinstance(tool_call, dict):
+                func = tool_call.get("function") or {}
+                tokens += self.count_text(coerce_countable_text(func.get("name")))
+                tokens += self.count_text(coerce_countable_text(func.get("arguments")))
 
         return tokens
 
diff --git a/tests/test_providers/test_anthropic.py b/tests/test_providers/test_anthropic.py
index 63fa1f73f..833a6ffc9 100644
--- a/tests/test_providers/test_anthropic.py
+++ b/tests/test_providers/test_anthropic.py
@@ -54,6 +54,18 @@ class TestAnthropicTokenCounting:
         count = counter.count_messages(messages)
         assert count > 0
 
+    def test_count_messages_tolerates_null_tool_calls(self, anthropic_provider):
+        # OpenAI-format assistant messages routinely carry `tool_calls: null`
+        # (and occasionally `function: null`) on a no-tool turn. The estimated
+        # counter iterated the value after only a key-presence check, so it
+        # raised `TypeError: 'NoneType' object is not iterable`.
+        counter = anthropic_provider.get_token_counter("claude-3-5-sonnet-20241022")
+        messages = [
+            {"role": "assistant", "content": "hi", "tool_calls": None},
+            {"role": "assistant", "content": "x", "tool_calls": [{"id": "a", "function": None}]},
+        ]
+        assert counter.count_messages(messages) > 0
+
     def test_count_text_allows_literal_special_tokens(self, anthropic_provider):
         counter = anthropic_provider.get_token_counter("claude-3-5-sonnet-20241022")
         count = counter.count_text("prefix <|fim_suffix|> suffix")