diff --git a/embassy-usb/src/builder.rs b/embassy-usb/src/builder.rs
index 07b2a177c..305dfa02e 100644
--- a/embassy-usb/src/builder.rs
+++ b/embassy-usb/src/builder.rs
@@ -346,7 +346,7 @@ impl<'a, 'd, D: Driver<'d>> FunctionBuilder<'a, 'd, D> {
         }
 
         if !self.builder.msos_descriptor.is_in_function_subset() {
-            self.builder.msos_descriptor.function(self.first_interface.0);
+            self.builder.msos_descriptor.function(self.first_interface);
         }
 
         #[cfg(feature = "msos-descriptor")]
diff --git a/embassy-usb/src/msos.rs b/embassy-usb/src/msos.rs
index 19ed34979..b1e0335ee 100644
--- a/embassy-usb/src/msos.rs
+++ b/embassy-usb/src/msos.rs
@@ -7,6 +7,7 @@
 use core::mem::size_of;
 
 use super::{capability_type, BosWriter};
+use crate::types::InterfaceNumber;
 
 /// A serialized Microsoft OS 2.0 Descriptor set.
 ///
@@ -125,7 +126,7 @@ impl<'d> MsOsDescriptorWriter<'d> {
     }
 
     /// Add a function subset.
-    pub fn function(&mut self, first_interface: u8) {
+    pub fn function(&mut self, first_interface: InterfaceNumber) {
         assert!(
             self.config_mark.is_some(),
             "MsOsDescriptorWriter: function subset requires a configuration subset"
@@ -376,14 +377,14 @@ impl DescriptorSet for ConfigurationSubsetHeader {
 pub struct FunctionSubsetHeader {
     wLength: u16,
     wDescriptorType: u16,
-    bFirstInterface: u8,
+    bFirstInterface: InterfaceNumber,
     bReserved: u8,
     wSubsetLength: u16,
 }
 
 impl FunctionSubsetHeader {
     /// Creates a function subset header
-    pub fn new(first_interface: u8) -> Self {
+    pub fn new(first_interface: InterfaceNumber) -> Self {
         FunctionSubsetHeader {
             wLength: (size_of::<Self>() as u16).to_le(),
             wDescriptorType: (Self::TYPE as u16).to_le(),
diff --git a/embassy-usb/src/types.rs b/embassy-usb/src/types.rs
index 15d195002..c7a47f7e4 100644
--- a/embassy-usb/src/types.rs
+++ b/embassy-usb/src/types.rs
@@ -3,6 +3,7 @@
 /// A handle for a USB interface that contains its number.
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
 #[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[repr(transparent)]
 pub struct InterfaceNumber(pub u8);
 
 impl InterfaceNumber {
@@ -20,6 +21,7 @@ impl From<InterfaceNumber> for u8 {
 /// A handle for a USB string descriptor that contains its index.
 #[derive(Copy, Clone, Eq, PartialEq)]
 #[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[repr(transparent)]
 pub struct StringIndex(pub u8);
 
 impl StringIndex {
diff --git a/examples/nrf52840/src/bin/usb_serial_winusb.rs b/examples/nrf52840/src/bin/usb_serial_winusb.rs
index ade6af527..6561fc3b4 100644
--- a/examples/nrf52840/src/bin/usb_serial_winusb.rs
+++ b/examples/nrf52840/src/bin/usb_serial_winusb.rs
@@ -12,6 +12,7 @@ use embassy_nrf::{interrupt, pac};
 use embassy_usb::class::cdc_acm::{CdcAcmClass, State};
 use embassy_usb::driver::EndpointError;
 use embassy_usb::msos::{self, windows_version};
+use embassy_usb::types::InterfaceNumber;
 use embassy_usb::{Builder, Config};
 use {defmt_rtt as _, panic_probe as _};
 
@@ -77,7 +78,7 @@ async fn main(_spawner: Spawner) {
     // Inside a class constructor, you would just need to call `FunctionBuilder::msos_feature` instead.
     let msos_writer = builder.msos_writer();
     msos_writer.configuration(0);
-    msos_writer.function(0);
+    msos_writer.function(InterfaceNumber(0));
     msos_writer.function_feature(msos::CompatibleIdFeatureDescriptor::new("WINUSB", ""));
     msos_writer.function_feature(msos::RegistryPropertyFeatureDescriptor::new(
         "DeviceInterfaceGUIDs",