diff --git a/src/__tests__/format.spec.ts b/src/__tests__/format.spec.ts
index 89c972e..85f5c7c 100644
--- a/src/__tests__/format.spec.ts
+++ b/src/__tests__/format.spec.ts
@@ -271,4 +271,13 @@ describe("format with a timezone", () => {
       })
     ).toBe("12/19/89, 1:30:10 AM -0600")
   })
+  it("does not throw when locale already contains a Unicode extension subtag (#68)", () => {
+    // ar-u-nu-arab requests Arabic-Indic numerals via BCP 47 Unicode extension.
+    // Previously, format() appended "-u-hc-h23" producing the invalid tag
+    // "ar-u-nu-arab-u-hc-h23" which throws "Invalid language tag".
+    expect(() => format("2024-01-15", "YYYY/MM/DD", "ar-u-nu-arab")).not.toThrow()
+    // The formatted output should use Arabic-Indic digits (٢٠٢٤, etc.)
+    const result = format("2024-01-15", "YYYY/MM/DD", "ar-u-nu-arab")
+    expect(result).toBe("٢٠٢٤/٠١/١٥")
+  })
 })
diff --git a/src/common.ts b/src/common.ts
index 92d911d..f97f3f9 100644
--- a/src/common.ts
+++ b/src/common.ts
@@ -248,7 +248,14 @@ function createPartMap(
   const genitiveParts: Part[] = []
 
   function addValues(requestedParts: Part[], hour12 = false) {
-    const preciseLocale = `${locale}-u-hc-${hour12 ? "h12" : "h23"}`
+    // Use Intl.Locale to merge the hourCycle extension into the locale tag.
+    // Plain string concatenation (e.g. `${locale}-u-hc-h23`) produces an
+    // invalid BCP 47 tag when the locale already contains a Unicode extension
+    // block (e.g. "ar-u-nu-arab"), because a tag may only have one "-u-" block.
+    // Intl.Locale merges extensions correctly into a single "-u-" subtag.
+    const preciseLocale = new Intl.Locale(locale, {
+      hourCycle: hour12 ? "h12" : "h23",
+    }).toString()
     valueParts.push(
       ...new Intl.DateTimeFormat(
         preciseLocale,