Reading an interval that has months as a TimeSpan throws InvalidCastException, because a month has no fixed length (#4599). The check in TimeSpanIntervalConverter.Read is months > 0, though, so a negative month count gets past it and the months are dropped:

// SELECT INTERVAL '1 month'   -> InvalidCastException, as intended
// SELECT INTERVAL '-1 month'  -> TimeSpan.Zero, no exception
var value = reader.GetFieldValue<TimeSpan>(0);

This changes the check to months != 0, which is what DurationConverter in Npgsql.NodaTime already does for the same case.

Interval_with_negative_months_cannot_read_as_TimeSpan sits next to the existing Interval_with_months_cannot_read_as_TimeSpan and fails without the fix. The DateTimeTests and NodaTime tests pass against PostgreSQL 17 (155).

Found with the pengdows ADO.NET conformance suite, whose type-fidelity tests round-trip each type's values through every driver. Npgsql 4 and 5 read -1 month as -30 days; 6 through current main read it as zero.

🤖 Generated with Claude Code