diff --git a/sw/host/opentitanlib/src/transport/errors.rs b/sw/host/opentitanlib/src/transport/errors.rs
index 327389d..0e6a7c2 100644
--- a/sw/host/opentitanlib/src/transport/errors.rs
+++ b/sw/host/opentitanlib/src/transport/errors.rs
@@ -20,6 +20,8 @@
     UsbGenericError(String),
     #[error("Error opening USB device: {0}")]
     UsbOpenError(String),
+    #[error("Transport does not support {0:?}")]
+    InvalidInterface(TransportInterfaceType),
     #[error("Transport does not support {0:?} instance {1}")]
     InvalidInstance(TransportInterfaceType, String),
     #[error("Encountered non-unicode device path")]
diff --git a/sw/host/opentitanlib/src/transport/mod.rs b/sw/host/opentitanlib/src/transport/mod.rs
index 2afd2ec..3f0f6f1 100644
--- a/sw/host/opentitanlib/src/transport/mod.rs
+++ b/sw/host/opentitanlib/src/transport/mod.rs
@@ -79,23 +79,23 @@
 
     /// Returns a SPI [`Target`] implementation.
     fn spi(&self, _instance: &str) -> Result<Rc<dyn Target>> {
-        unimplemented!();
+        Err(TransportError::InvalidInterface(TransportInterfaceType::Spi))
     }
     /// Returns a I2C [`Bus`] implementation.
     fn i2c(&self, _instance: &str) -> Result<Rc<dyn Bus>> {
-        unimplemented!();
+        Err(TransportError::InvalidInterface(TransportInterfaceType::I2c))
     }
     /// Returns a [`Uart`] implementation.
     fn uart(&self, _instance: &str) -> Result<Rc<dyn Uart>> {
-        unimplemented!();
+        Err(TransportError::InvalidInterface(TransportInterfaceType::Uart))
     }
     /// Returns a [`GpioPin`] implementation.
     fn gpio_pin(&self, _instance: &str) -> Result<Rc<dyn GpioPin>> {
-        unimplemented!();
+        Err(TransportError::InvalidInterface(TransportInterfaceType::Gpio))
     }
     /// Invoke non-standard functionality of some Transport implementations.
     fn dispatch(&self, _action: &dyn Any) -> Result<Option<Box<dyn Serialize>>> {
-        Err(TransportError::UnsupportedOperation.into())
+        Err(TransportError::UnsupportedOperation)
     }
 }