diff --git a/tests/test_formatters_fields.py b/tests/test_formatters_fields.py
new file mode 100644
index 0000000..e428a13
--- /dev/null
+++ b/tests/test_formatters_fields.py
@@ -0,0 +1,72 @@
+"""Optional fields and value handling in the JSON formatters (issue #137)."""
+import json
+import logging
+
+import pytest
+
+import json_logging
+from json_logging.formatters import BaseJSONFormatter, JSONLogFormatter
+
+
+@pytest.fixture
+def component_settings():
+    """Restore the module level component settings, which are read when a formatter is constructed."""
+    saved = (json_logging.COMPONENT_ID, json_logging.COMPONENT_NAME, json_logging.COMPONENT_INSTANCE_INDEX)
+    # base_object_common is a class level dict that formatter construction writes into, so restore it as well
+    saved_common = dict(BaseJSONFormatter.base_object_common)
+    for key in ("component_id", "component_name", "component_instance_idx"):
+        BaseJSONFormatter.base_object_common.pop(key, None)
+    yield
+    BaseJSONFormatter.base_object_common.clear()
+    BaseJSONFormatter.base_object_common.update(saved_common)
+    json_logging.COMPONENT_ID, json_logging.COMPONENT_NAME, json_logging.COMPONENT_INSTANCE_INDEX = saved
+
+
+def _format(**extra):
+    record = logging.LogRecord("formatter-test", logging.INFO, __file__, 1, "hello", None, None)
+    for key, value in extra.items():
+        setattr(record, key, value)
+    return json.loads(JSONLogFormatter().format(record))
+
+
+def test_component_settings_are_added_when_set(component_settings):
+    json_logging.COMPONENT_ID = "id-1"
+    json_logging.COMPONENT_NAME = "name-1"
+    json_logging.COMPONENT_INSTANCE_INDEX = 2
+
+    output = _format()
+
+    assert output["component_id"] == "id-1"
+    assert output["component_name"] == "name-1"
+    assert output["component_instance_idx"] == 2
+
+
+def test_component_settings_are_omitted_by_default(component_settings):
+    output = _format()
+
+    assert "component_id" not in output
+    assert "component_name" not in output
+    assert "component_instance_idx" not in output
+
+
+def test_non_serialisable_extra_value_falls_back_to_repr():
+    class Unserialisable:
+        def __repr__(self):
+            return "<unserialisable>"
+
+    output = _format(thing=Unserialisable())
+
+    assert output["thing"] == "<unserialisable>"
+
+
+def test_props_dict_is_merged_into_the_root_of_the_log():
+    output = _format(props={"user": "alice", "attempt": 3})
+
+    assert output["user"] == "alice"
+    assert output["attempt"] == 3
+
+
+def test_props_that_is_not_a_dict_is_not_merged():
+    output = _format(props="not-a-dict")
+
+    assert "not-a-dict" not in output.values()
diff --git a/tests/test_init_validation.py b/tests/test_init_validation.py
new file mode 100644
index 0000000..3323878
--- /dev/null
+++ b/tests/test_init_validation.py
@@ -0,0 +1,75 @@
+"""Argument validation and error paths of the public initialisation API (issue #137)."""
+import logging
+
+import pytest
+from helpers.imports import undo_imports_from_package
+
+
+@pytest.fixture
+def fresh_json_logging():
+    """Import a pristine json_logging so module level init state does not leak between tests."""
+    undo_imports_from_package("json_logging")
+    import json_logging
+    yield json_logging
+    undo_imports_from_package("json_logging")
+
+
+def test_init_framework_twice_raises(fresh_json_logging):
+    fresh_json_logging.init_flask()
+    with pytest.raises(RuntimeError, match="more than once"):
+        fresh_json_logging.init_flask()
+
+
+def test_init_rejects_custom_formatter_that_is_not_a_formatter(fresh_json_logging):
+    with pytest.raises(ValueError, match="custom_formatter"):
+        fresh_json_logging.init_non_web(custom_formatter=dict)
+
+
+def test_init_rejects_unknown_framework(fresh_json_logging):
+    with pytest.raises(RuntimeError, match="not a supported framework"):
+        fresh_json_logging.init_non_web(framework_name="no-such-framework")
+
+
+def test_init_request_instrument_before_init_raises(fresh_json_logging):
+    with pytest.raises(RuntimeError, match="init the logging first"):
+        fresh_json_logging.init_request_instrument(app=object())
+
+
+def test_init_request_instrument_after_non_web_init_raises(fresh_json_logging):
+    fresh_json_logging.init_non_web()
+    with pytest.raises(RuntimeError, match="init the logging first"):
+        fresh_json_logging.init_request_instrument(app=object())
+
+
+def test_init_request_instrument_rejects_bad_custom_formatter(fresh_json_logging):
+    fresh_json_logging.init_flask()
+    with pytest.raises(ValueError, match="custom_formatter"):
+        fresh_json_logging.init_request_instrument(app=object(), custom_formatter=dict)
+
+
+def test_init_request_instrument_rejects_bad_request_response_dto_class(fresh_json_logging):
+    fresh_json_logging.init_flask()
+    with pytest.raises(ValueError, match="request_response_dto_class"):
+        fresh_json_logging.init_request_instrument(app=object(), request_response_dto_class=dict)
+
+
+def test_get_request_logger_before_init_raises(fresh_json_logging):
+    with pytest.raises(RuntimeError, match="only available"):
+        fresh_json_logging.get_request_logger()
+
+
+def test_config_root_logger_without_handlers_logs_error_and_returns(fresh_json_logging, monkeypatch):
+    fresh_json_logging.ENABLE_JSON_LOGGING = True
+    monkeypatch.setattr(logging.root, "handlers", [])
+    errors = []
+    monkeypatch.setattr(fresh_json_logging._logger, "error", lambda msg, *a, **kw: errors.append(msg))
+
+    def fail(*args, **kwargs):
+        raise AssertionError("root logger must not be touched when it has no handlers")
+
+    monkeypatch.setattr(fresh_json_logging.util, "update_formatter_for_loggers", fail)
+
+    fresh_json_logging.config_root_logger()
+
+    assert len(errors) == 1
+    assert "No logging handlers found" in errors[0]
diff --git a/tests/test_util_validation.py b/tests/test_util_validation.py
new file mode 100644
index 0000000..082ded3
--- /dev/null
+++ b/tests/test_util_validation.py
@@ -0,0 +1,43 @@
+"""Error paths of the helpers in json_logging.util (issue #137)."""
+import logging
+
+import pytest
+
+from json_logging import util
+
+
+def test_update_formatter_for_loggers_rejects_non_logger():
+    with pytest.raises(RuntimeError):
+        util.update_formatter_for_loggers(["not-a-logger"], logging.Formatter)
+
+
+def test_update_formatter_for_loggers_replaces_other_formatters_only():
+    class Wanted(logging.Formatter):
+        pass
+
+    logger = logging.Logger("util-validation-test")
+    replaced = logging.StreamHandler()
+    kept = logging.StreamHandler()
+    already_wanted = Wanted()
+    kept.setFormatter(already_wanted)
+    logger.addHandler(replaced)
+    logger.addHandler(kept)
+
+    util.update_formatter_for_loggers([logger], Wanted)
+
+    assert isinstance(replaced.formatter, Wanted)
+    assert kept.formatter is already_wanted
+
+
+def test_validate_subclass_accepts_subclass():
+    assert util.validate_subclass(logging.StreamHandler, logging.Handler) is True
+
+
+def test_validate_subclass_rejects_non_subclass():
+    with pytest.raises(RuntimeError, match="is not a subclass of"):
+        util.validate_subclass(dict, logging.Handler)
+
+
+@pytest.mark.parametrize("value, default, expected", [("7", 1, 7), ("x", 1, 1), (None, 3, 3), (4, 0, 4)])
+def test_parse_int_falls_back_to_default(value, default, expected):
+    assert util.parse_int(value, default) == expected