What causes "Unsupported grammatical case." in Humanizr/Humanizer?

Validate with Enum.IsDefined(typeof(GrammaticalCase), grammaticalCase) before invoking the case-aware path.

Humanizr/Humanizer · error · ArgumentOutOfRangeException

Unsupported grammatical case.

Error message

Unsupported grammatical case.

What it means

Thrown in IGrammaticalCaseTimeSpanFormatter.TimeSpanHumanize when the grammaticalCase argument is not a defined member of the GrammaticalCase enum. The guard casts to uint and compares against (uint)GrammaticalCase.Causal (the last defined member), so any value above Causal or a negative cast-to-large-uint fails. The exception is ArgumentOutOfRangeException named 'grammaticalCase' with the offending value in the message.

Source

Thrown at src/Humanizer/Localisation/Formatters/DefaultFormatter.cs:144

        }

        return category;
    }

    string IGrammaticalCaseTimeSpanFormatter.TimeSpanHumanize(
        TimeUnit timeUnit,
        int unit,
        GrammaticalCase grammaticalCase)
    {
        if (GetType().Assembly != typeof(DefaultFormatter).Assembly)
        {
            throw new NotSupportedException(
                $"Custom formatter type '{GetType().FullName}' must explicitly implement {nameof(IGrammaticalCaseTimeSpanFormatter)} to support grammatical-case-aware durations.");
        }

        if ((uint)grammaticalCase > (uint)GrammaticalCase.Causal)
        {
            throw new ArgumentOutOfRangeException(nameof(grammaticalCase), grammaticalCase, "Unsupported grammatical case.");
        }

        if ((uint)timeUnit > (uint)TimeUnit.Year)
        {
            throw new ArgumentOutOfRangeException(nameof(timeUnit), timeUnit, "Unsupported time unit.");
        }

        var table = LocaleDurationCaseTableCatalog.Resolve(Culture)
            ?? throw new NotSupportedException(
                $"Culture '{Culture.Name}' has no grammatical-case classification for duration phrases.");

        if (table.Classification == LocaleDurationCaseClassification.Unsupported)
        {
            throw new NotSupportedException(
                $"Culture '{Culture.Name}' has an applicable grammatical case system, but verified duration forms are unavailable.");
        }

        if (table.Classification == LocaleDurationCaseClassification.NotApplicable)

View on GitHub (pinned to ffc2b77c0f)

Solutions

Validate with Enum.IsDefined(typeof(GrammaticalCase), grammaticalCase) before invoking the case-aware path.
Parse the case from strings via Enum.TryParse<GrammaticalCase> and reject unknown tokens.
When you only need the nominative form, use the non-case-aware TimeSpanHumanize(timeUnit, unit) overload.

Example fix

// before
var text = formatter.TimeSpanHumanize(unit, count, (GrammaticalCase)caseCode);

// after
if (!Enum.IsDefined(typeof(GrammaticalCase), caseCode))
    throw new ArgumentOutOfRangeException(nameof(caseCode));
var text = formatter.TimeSpanHumanize(unit, count, (GrammaticalCase)caseCode);

Defensive patterns

Strategy: validation

Validate before calling

if (!Enum.IsDefined(typeof(GrammaticalCase), grammaticalCase))
    throw new ArgumentOutOfRangeException(nameof(grammaticalCase));
var text = caseFormatter.TimeSpanHumanize(timeUnit, unit, grammaticalCase);

Type guard

static bool IsValidGrammaticalCase(GrammaticalCase c) => Enum.IsDefined(typeof(GrammaticalCase), c);

Try / catch

try { return caseFormatter.TimeSpanHumanize(timeUnit, unit, grammaticalCase); }
catch (ArgumentOutOfRangeException ex) when (ex.ParamName == "grammaticalCase")
{ return caseFormatter.TimeSpanHumanize(timeUnit, unit, GrammaticalCase.Nominative); }

Prevention

Validate GrammaticalCase at the boundary with Enum.IsDefined.
Parse from strings with Enum.TryParse<GrammaticalCase>.
Prefer the non-case-aware overload when you only need the nominative form.

When it happens

Trigger: Calling the case-aware formatter with (GrammaticalCase)999; passing a GrammaticalCase deserialized from an out-of-range integer; default(GrammaticalCase) is Nominative (0) and is valid so it will NOT throw.

Common situations: Configuration/serialization storing the case as a raw int; interop with another enum cast to GrammaticalCase; reflection-based callers that construct the value dynamically.

Related errors

AI-assisted analysis of Humanizr/Humanizer@ffc2b77c0f (2026-08-13). Data as JSON: /api/errors/67c0aa4b567e1a0f. Report an issue: GitHub.