diff --git a/crates/rolldown_utils/src/xxhash.rs b/crates/rolldown_utils/src/xxhash.rs
--- a/crates/rolldown_utils/src/xxhash.rs
+++ b/crates/rolldown_utils/src/xxhash.rs
@@ -18,12 +18,25 @@ pub fn xxhash_with_base(input: &[u8], base: u8) -> String {
   let hash = if input.len() == 16 { input } else { &xxh3_128(input).to_le_bytes() };

   let chars = match base {
     64 => CHARACTERS_BASE64,
     36 => &CHARACTERS_BASE64[26..(26 + 36)],
     16 => CHARACTERS_BASE16,
     _ => {
       unreachable!()
     }
   };

-  to_string(hash, base, chars).unwrap()
+  let mut result = to_string(hash, base, chars).unwrap();
+  // `base_encode::to_string` produces variable-length output like decimal
+  // (no leading-zero padding). Left-pad with the zero-digit to the maximum
+  // length for a 128-bit value so that `&hash[..placeholder.len()]` in
+  // finalize_chunks.rs never panics on a shorter-than-expected string.
+  let max_len = match base {
+    64 => 22, // ceil(128 / log2(64))
+    36 => 25, // ceil(128 / log2(36))
+    16 => 32, // 128 / 4
+    _ => unreachable!(),
+  };
+  while result.len() < max_len {
+    result.insert(0, chars[0] as char);
+  }
+  result
 }

 #[test]
 fn test_xxhash_with_base() {
-  assert_eq!(&xxhash_with_base(b"hello", 64), "YOFJeqs95x38-Gwetwem1");
+  assert_eq!(&xxhash_with_base(b"hello", 64), "AYOFJeqs95x38-Gwetwem1");
   assert_eq!(&xxhash_with_base(b"hello", 36), "bpwli5k6mqm0gij09mxrh9npj");
   assert_eq!(&xxhash_with_base(b"hello", 16), "1838525eaacf79c77f3e1b07adc1e9b5");
 }