diff --git a/platform/src/register.rs b/platform/src/register.rs
index 976653a5..0afde2f1 100644
--- a/platform/src/register.rs
+++ b/platform/src/register.rs
@@ -1,3 +1,5 @@
+use core::mem::transmute;
+
 /// In order to work with Miri's `-Zmiri-track-raw-pointers` flag, we cannot
 /// pass pointers to the kernel through `usize` values (as casting to and from
 /// `usize` drops the pointer`s tag). Instead, `RawSyscalls` uses the `Register`
@@ -16,19 +18,28 @@ pub struct Register(pub *mut ());
 
 impl From<crate::ErrorCode> for Register {
     fn from(value: crate::ErrorCode) -> Register {
-        Register(value as u16 as *mut ())
+        (value as usize).into()
     }
 }
 
 impl From<u32> for Register {
     fn from(value: u32) -> Register {
-        Register(value as *mut ())
+        (value as usize).into()
     }
 }
 
 impl From<usize> for Register {
     fn from(value: usize) -> Register {
-        Register(value as *mut ())
+        // Note: clippy is wrong here; transmute has different semantics than
+        // `as` casts under strict provenance.
+        #[allow(clippy::useless_transmute)]
+        // We want to convert using the same semantics as core::ptr::invalid:
+        // convert the usize into a pointer with that address without attaching
+        // provenance to it. However, core::ptr::invalid is a nightly-only
+        // function. In order to build on stable, we copy its implementation.
+        // Safety: Raw pointers do not have any validity invariants that usize
+        // does not have; a raw pointer can point to any address.
+        Register(unsafe { transmute(value) })
     }
 }
 
diff --git a/rust-toolchain b/rust-toolchain
index 4c36818b..873e2ff6 100644
--- a/rust-toolchain
+++ b/rust-toolchain
@@ -1,7 +1,7 @@
 [toolchain]
 # See https://rust-lang.github.io/rustup-components-history/ for a list of
 # recently nightlies and what components are available for them.
-channel = "nightly-2022-06-10"
+channel = "nightly-2023-08-22"
 components = ["clippy", "miri", "rustfmt"]
 targets = ["thumbv6m-none-eabi",
            "thumbv7em-none-eabi",
diff --git a/unittest/Cargo.toml b/unittest/Cargo.toml
index 3a7eb5f0..094f0915 100644
--- a/unittest/Cargo.toml
+++ b/unittest/Cargo.toml
@@ -10,4 +10,4 @@ version = "0.1.0"
 
 [dependencies]
 libtock_platform = { path = "../platform" }
-thiserror = "1.0"
+thiserror = "1.0.44"