diff --git a/sqlite-compress/src/functions/header.rs b/sqlite-compress/src/functions/header.rs
index 2a26aaf..416a984 100644
--- a/sqlite-compress/src/functions/header.rs
+++ b/sqlite-compress/src/functions/header.rs
@@ -28,10 +28,12 @@ impl TryFrom<u8> for CodecKind {
 pub struct Header {
     /// Version of the prefix format understood by this decoder.
     pub version: u8,
-    /// Compression algorithm used for the payload.
-    pub codec: u8,
     /// Number of bytes occupied by the full prefix, including future fields.
     pub header_len: u8,
+    /// Compression algorithm used for the payload.
+    pub codec: u8,
+    /// Explicit padding to ensure 4-byte alignment.
+    pub padding: u8,
     /// Dictionary used for compression; zero denotes a raw Zstandard payload.
     pub dict_id: U32,
     /// Original byte length used to bound decompression output.
@@ -54,8 +56,9 @@ impl Header {
     pub fn new(dict_id: DictId, uncompressed_len: u32) -> Self {
         Self {
             version: Self::VERSION,
-            codec: CodecKind::Zstd as u8,
             header_len: Self::PREFIX_SIZE as u8,
+            codec: CodecKind::Zstd as u8,
+            padding: 0,
             dict_id: U32::new(dict_id.get()),
             uncompressed_len: U32::new(uncompressed_len),
         }
diff --git a/sqlite-compress/tests/dict_train.rs b/sqlite-compress/tests/dict_train.rs
index 4ac7744..7ba29fa 100644
--- a/sqlite-compress/tests/dict_train.rs
+++ b/sqlite-compress/tests/dict_train.rs
@@ -33,7 +33,7 @@ pub fn decompress_blob(state: &ExtensionState, blob: &[u8]) -> Vec<u8> {
 }
 
 pub fn header_dict_id(blob: &[u8]) -> u32 {
-    u32::from_le_bytes(blob[3..7].try_into().expect("framed header"))
+    u32::from_le_bytes(blob[4..8].try_into().expect("framed header"))
 }
 
 #[test]