diff --git a/Cargo.lock b/Cargo.lock
index a7ac948b638..10110f599b5 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1956,6 +1956,7 @@ dependencies = [
  "litemap",
  "log",
  "serde",
+ "writeable",
  "zerovec",
 ]
 
diff --git a/experimental/transliterate/Cargo.toml b/experimental/transliterate/Cargo.toml
index 5ede21f4692..e84b1d879e0 100644
--- a/experimental/transliterate/Cargo.toml
+++ b/experimental/transliterate/Cargo.toml
@@ -24,6 +24,7 @@ icu_provider = { workspace = true, features = ["macros", "experimental"] }
 icu_locid = { workspace = true }
 icu_collections = { workspace = true }
 icu_normalizer = { workspace = true }
+writeable = { workspace = true }
 
 icu_properties = { workspace = true, optional = true }
 icu_unicodeset_parse = { workspace = true, optional = true }
diff --git a/experimental/transliterate/src/ids.rs b/experimental/transliterate/src/ids.rs
index 7462091e8b0..830f7dcf065 100644
--- a/experimental/transliterate/src/ids.rs
+++ b/experimental/transliterate/src/ids.rs
@@ -2,19 +2,34 @@
 // called LICENSE at the top level of the ICU4X source tree
 // (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).
 
-use alloc::string::ToString;
 use icu_locid::Locale;
 use icu_provider::prelude::*;
+use writeable::Writeable;
 
 pub fn bcp47_to_data_locale(locale: &Locale) -> DataLocale {
     let mut data_locale = DataLocale::default();
     #[allow(clippy::unwrap_used)] // any BCP-47 locale is a valid aux key
-    data_locale.set_aux(locale.to_string().parse().unwrap());
+    data_locale.set_aux(
+        AuxiliaryKeys::try_from_iter(
+            locale
+                .write_to_string()
+                .split('-')
+                .map(str::parse)
+                .filter_map(Result::ok),
+        )
+        .unwrap(),
+    );
     data_locale
 }
 
 pub fn unparsed_bcp47_to_data_locale(dep: &str) -> Result<DataLocale, DataError> {
     let mut data_locale = DataLocale::default();
-    data_locale.set_aux(dep.parse()?);
+    data_locale.set_aux(AuxiliaryKeys::try_from_iter(
+        dep.split('-')
+            .map(str::parse)
+            // TODO: Bubble a potential subtag parse error out. Currently
+            // we ignore it inside of the `filter_map`.
+            .filter_map(Result::ok),
+    )?);
     Ok(data_locale)
 }
diff --git a/provider/core/src/request.rs b/provider/core/src/request.rs
index caf1da97171..8889f48a499 100644
--- a/provider/core/src/request.rs
+++ b/provider/core/src/request.rs
@@ -959,7 +959,15 @@ impl AuxiliaryKeys {
     pub fn iter(&self) -> impl Iterator<Item = Subtag> + '_ {
         self.value
             .split(Self::separator())
-            .flat_map(|x| x.parse().ok())
+            .filter_map(|x| {
+                match x.parse() {
+                    Ok(x) => Some(x),
+                    Err(_) => {
+                        debug_assert!(false, "failed to convert to subtag: {x}");
+                        None
+                    }
+                }
+            })
     }
 
     pub(crate) fn strict_cmp_iter<'l, I>(&self, mut subtags: I) -> SubtagOrderingResult<I>