# Exceptions

Instro exception classes.

| [`InstroError`](https://dev.nominal.io/instro/ref/library/instro.lib.exceptions.InstroError.md#instro.lib.exceptions.InstroError "instro.lib.exceptions.InstroError") | Base Instro error. |
| --- | --- |
| [`FeatureNotSupportedError`](https://dev.nominal.io/instro/ref/library/instro.lib.exceptions.FeatureNotSupportedError.md#instro.lib.exceptions.FeatureNotSupportedError "instro.lib.exceptions.FeatureNotSupportedError") | Unsupported feature error. |
| [`InstrumentNotOpenError`](https://dev.nominal.io/instro/ref/library/instro.lib.exceptions.InstrumentNotOpenError.md#instro.lib.exceptions.InstrumentNotOpenError "instro.lib.exceptions.InstrumentNotOpenError") | Raised when an instrument method is called before open(). |
| [`UnknownHolderError`](https://dev.nominal.io/instro/ref/library/instro.lib.exceptions.UnknownHolderError.md#instro.lib.exceptions.UnknownHolderError "instro.lib.exceptions.UnknownHolderError") | Raised when a transport is closed with an object that never opened it. |