diff --git a/api/libs/json_in_md_parser.py b/api/libs/json_in_md_parser.py
index a59fd1eb805798..1619e79928b11d 100644
--- a/api/libs/json_in_md_parser.py
+++ b/api/libs/json_in_md_parser.py
@@ -56,6 +56,12 @@ def parse_and_check_json_markdown(text: str, expected_keys: list[str]):
             json_obj = json_obj[0]
         else:
             raise OutputParserError(f"got invalid return object. obj:{json_obj}")
+    if not isinstance(json_obj, dict):
+        # Scalars or plain strings (e.g. a fenced number/string) are not a
+        # valid return object. Without this check, `key not in json_obj`
+        # raises TypeError for numbers/None/bools and silently does a
+        # substring match for strings.
+        raise OutputParserError(f"got invalid return object. obj:{json_obj}")
     for key in expected_keys:
         if key not in json_obj:
             raise OutputParserError(
diff --git a/api/tests/unit_tests/libs/test_json_in_md_parser.py b/api/tests/unit_tests/libs/test_json_in_md_parser.py
index 48b6f749a83a9c..9535a6e72f5f26 100644
--- a/api/tests/unit_tests/libs/test_json_in_md_parser.py
+++ b/api/tests/unit_tests/libs/test_json_in_md_parser.py
@@ -124,3 +124,16 @@ def test_parse_json_markdown_backtick_in_surrounding_prose() -> None:
 def test_parse_json_markdown_fenced_scalar_still_supported() -> None:
     """Fenced content without brackets still parses via the fence fallback."""
     assert parse_json_markdown('```json\n"hello"\n```') == "hello"
+
+
+def test_parse_and_check_json_markdown_fenced_scalar_raises_output_parser_error() -> None:
+    """A valid JSON scalar (number/null/bool) must raise OutputParserError, not TypeError."""
+    for src in ("```json\n42\n```", "```json\nnull\n```", "```json\ntrue\n```"):
+        with pytest.raises(OutputParserError):
+            parse_and_check_json_markdown(src, ["answer"])
+
+
+def test_parse_and_check_json_markdown_rejects_non_dict_object() -> None:
+    """A JSON string that merely contains an expected key as a substring must not pass."""
+    with pytest.raises(OutputParserError):
+        parse_and_check_json_markdown('```json\n"the generated prompt text"\n```', ["prompt"])