QuantConnect/Lean · error · AssertionError

{call} should have returned {expected} ({type(expected)}) bu

Error message

{call} should have returned {expected} ({type(expected)}) but returned null

What it means

Same check_parameter helper, expected-not-null branch (lines 38-39). It asserts that get_parameter returns a value (either the configured value or the supplied default) instead of None. A None return here means parameter lookup stopped applying defaults or stopped reading existing keys like 'ema-fast'.

Source

Thrown at Algorithm.Python/GetParameterRegressionAlgorithm.py:39

    def initialize(self):
        self.set_start_date(2013, 10, 7)
        self.check_parameter(None, self.get_parameter("non-existing"), "GetParameter(\"non-existing\")")
        self.check_parameter("100", self.get_parameter("non-existing", "100"), "GetParameter(\"non-existing\", \"100\")")
        self.check_parameter(100, self.get_parameter("non-existing", 100), "GetParameter(\"non-existing\", 100)")
        self.check_parameter(100.0, self.get_parameter("non-existing", 100.0), "GetParameter(\"non-existing\", 100.0)")

        self.check_parameter("10", self.get_parameter("ema-fast"), "GetParameter(\"ema-fast\")")
        self.check_parameter(10, self.get_parameter("ema-fast", 100), "GetParameter(\"ema-fast\", 100)")
        self.check_parameter(10.0, self.get_parameter("ema-fast", 100.0), "GetParameter(\"ema-fast\", 100.0)")

        self.quit()

    def check_parameter(self, expected, actual, call):
        if expected == None and actual != None:
            raise AssertionError(f"{call} should have returned null but returned {actual} ({type(actual)})")

        if expected != None and actual == None:
            raise AssertionError(f"{call} should have returned {expected} ({type(expected)}) but returned null")

        if expected != None and actual != None and type(expected) != type(actual) or expected != actual:
            raise AssertionError(f"{call} should have returned {expected} ({type(expected)}) but returned {actual} ({type(actual)})")

View on GitHub (pinned to d2c3659f87)

Solutions

Verify the regression parameter config still defines ema-fast=10 (check Launcher/config and the regression test parameters).
Inspect GetParameter(name, defaultValue) overloads: when key is absent the default must be returned (coerced to the requested type), not null.
Confirm the algorithm receives its parameters dictionary during initialize (IAlgorithm.SetParameters / job parameters wiring).

Example fix

// before: default overload returns null
public int GetParameter(string name, int defaultValue) => null;
// after: return default when key missing
public int GetParameter(string name, int defaultValue)
    => _parameters.TryGetValue(name, out var v) && int.TryParse(v, out var i) ? i : defaultValue;

Defensive patterns

Strategy: validation

Validate before calling

# ensure a value or default before use
ema = self.get_parameter("ema-fast", 10)
if ema is None:
    raise RuntimeError("ema-fast parameter missing and no default applied")

Prevention

Always supply a default to get_parameter when the value is required.
Validate required parameters at the start of initialize and fail loudly.
Document expected parameter keys in the algorithm summary.

When it happens

Trigger: self.get_parameter("ema-fast") or self.get_parameter("non-existing", <default>) returns None. expected is non-None (the configured '10' / default), actual is None, so line 38-39 raises.

Common situations: The 'ema-fast' parameter was removed from the regression config; GetParameter default-argument overload regressed to always return null; parameters dictionary failed to load from the job/config.

Related errors

AI-assisted analysis of QuantConnect/Lean@d2c3659f87 (2026-08-13). Data as JSON: /api/errors/411b543988c2b4b3. Report an issue: GitHub.