diff --git a/src/claude_code_api/backend.py b/src/claude_code_api/backend.py
index 16dee85..b29c25a 100644
--- a/src/claude_code_api/backend.py
+++ b/src/claude_code_api/backend.py
@@ -398,12 +398,17 @@ def _block_to_dict(block: ContentBlock) -> dict[str, Any]:
             "input": block.input,
         }
     if isinstance(block, ToolResultBlock):
-        return {
+        # ``is_error`` is optional on the wire; emitting ``null`` makes the
+        # Anthropic API reject the message with "Input should be a valid
+        # boolean". Omit it entirely when we don't have a value.
+        result: dict[str, Any] = {
             "type": "tool_result",
             "tool_use_id": block.tool_use_id,
             "content": block.content,
-            "is_error": block.is_error,
         }
+        if block.is_error is not None:
+            result["is_error"] = block.is_error
+        return result
     if isinstance(block, ThinkingBlock):
         return {
             "type": "thinking",