diff --git a/pallas-primitives/src/byron/model.rs b/pallas-primitives/src/byron/model.rs
index 5e90622..4121b74 100644
--- a/pallas-primitives/src/byron/model.rs
+++ b/pallas-primitives/src/byron/model.rs
@@ -8,6 +8,7 @@ use pallas_crypto::hash::Hash;
 
 use crate::utils::{
     CborWrap, EmptyMap, KeyValuePairs, MaybeIndefArray, OrderPreservingProperties, TagWrap,
+    ZeroOrOneArray,
 };
 
 // Basic Cardano Types
@@ -612,46 +613,46 @@ impl minicbor::Encode for TxFeePol {
 #[derive(Debug, Encode, Decode)]
 pub struct BVerMod {
     #[n(0)]
-    pub script_version: (Option<u16>,),
+    pub script_version: ZeroOrOneArray<u16>,
 
     #[n(1)]
-    pub slot_duration: (Option<u64>,),
+    pub slot_duration: ZeroOrOneArray<u64>,
 
     #[n(2)]
-    pub max_block_size: (Option<u64>,),
+    pub max_block_size: ZeroOrOneArray<u64>,
 
     #[n(3)]
-    pub max_header_size: (Option<u64>,),
+    pub max_header_size: ZeroOrOneArray<u64>,
 
     #[n(4)]
-    pub max_tx_size: (Option<u64>,),
+    pub max_tx_size: ZeroOrOneArray<u64>,
 
     #[n(5)]
-    pub max_proposal_size: (Option<u64>,),
+    pub max_proposal_size: ZeroOrOneArray<u64>,
 
     #[n(6)]
-    pub mpc_thd: (Option<u64>,),
+    pub mpc_thd: ZeroOrOneArray<u64>,
 
     #[n(7)]
-    pub heavy_del_thd: (Option<u64>,),
+    pub heavy_del_thd: ZeroOrOneArray<u64>,
 
     #[n(8)]
-    pub update_vote_thd: (Option<u64>,),
+    pub update_vote_thd: ZeroOrOneArray<u64>,
 
     #[n(9)]
-    pub update_proposal_thd: (Option<u64>,),
+    pub update_proposal_thd: ZeroOrOneArray<u64>,
 
     #[n(10)]
-    pub update_implicit: (Option<u64>,),
+    pub update_implicit: ZeroOrOneArray<u64>,
 
     #[n(11)]
-    pub soft_fork_rule: (Option<(u64, u64, u64)>,),
+    pub soft_fork_rule: ZeroOrOneArray<(u64, u64, u64)>,
 
     #[n(12)]
-    pub tx_fee_policy: (Option<TxFeePol>,),
+    pub tx_fee_policy: ZeroOrOneArray<TxFeePol>,
 
     #[n(13)]
-    pub unlock_stake_epoch: (Option<EpochId>,),
+    pub unlock_stake_epoch: ZeroOrOneArray<EpochId>,
 }
 
 pub type UpData = (ByronHash, ByronHash, ByronHash, ByronHash);
@@ -668,7 +669,9 @@ pub struct UpProp {
     pub software_version: Option<(String, u32)>,
 
     #[n(3)]
-    pub data: Option<TagWrap<(String, UpData), 258>>,
+    // HACK: CDDL show a tag wrap 258, but chain data doesn't present the tag
+    //pub data: TagWrap<(String, UpData), 258>,
+    pub data: KeyValuePairs<String, UpData>,
 
     #[n(4)]
     pub attributes: Option<Attributes>,
@@ -698,7 +701,7 @@ pub struct UpVote {
 #[derive(Debug, Encode, Decode)]
 pub struct Up {
     #[n(0)]
-    pub proposal: Option<UpProp>,
+    pub proposal: ZeroOrOneArray<UpProp>,
 
     #[n(1)]
     pub votes: MaybeIndefArray<UpVote>,
@@ -959,6 +962,7 @@ mod tests {
             include_str!("test_data/test2.block"),
             include_str!("test_data/test3.block"),
             include_str!("test_data/test4.block"),
+            include_str!("test_data/test5.block"),
         ];
 
         for (idx, block_str) in test_blocks.iter().enumerate() {
diff --git a/pallas-primitives/src/byron/test_data/test5.block b/pallas-primitives/src/byron/test_data/test5.block
new file mode 100644
index 0000000..5016be6
--- /dev/null
+++ b/pallas-primitives/src/byron/test_data/test5.block
@@ -0,0 +1 @@
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
\ No newline at end of file
diff --git a/pallas-primitives/src/utils.rs b/pallas-primitives/src/utils.rs
index b9e08e2..b5bb668 100644
--- a/pallas-primitives/src/utils.rs
+++ b/pallas-primitives/src/utils.rs
@@ -318,3 +318,61 @@ impl minicbor::encode::Encode for EmptyMap {
         Ok(())
     }
 }
+
+/// An array with zero or one elements
+///
+/// A common pattern seen in the CDDL is to represent optional values as an
+/// array containing zero or more items. This structure reflects that pattern
+/// while providing semantic meaning.
+#[derive(Debug)]
+pub struct ZeroOrOneArray<T>(Option<T>);
+
+impl<T> Deref for ZeroOrOneArray<T> {
+    type Target = Option<T>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<'b, T> minicbor::decode::Decode<'b> for ZeroOrOneArray<T>
+where
+    T: Decode<'b>,
+{
+    fn decode(d: &mut minicbor::Decoder<'b>) -> Result<Self, minicbor::decode::Error> {
+        let len = d.array()?;
+
+        match len {
+            Some(0) => Ok(ZeroOrOneArray(None)),
+            Some(1) => Ok(ZeroOrOneArray(Some(d.decode()?))),
+            Some(_) => Err(minicbor::decode::Error::Message(
+                "found invalid len for zero-or-one pattern",
+            )),
+            None => Err(minicbor::decode::Error::Message(
+                "found invalid indefinite len array for zero-or-one pattern",
+            )),
+        }
+    }
+}
+
+impl<T> minicbor::encode::Encode for ZeroOrOneArray<T>
+where
+    T: minicbor::Encode,
+{
+    fn encode<W: minicbor::encode::Write>(
+        &self,
+        e: &mut minicbor::Encoder<W>,
+    ) -> Result<(), minicbor::encode::Error<W::Error>> {
+        match &self.0 {
+            Some(x) => {
+                e.array(1)?;
+                e.encode(x)?;
+            }
+            None => {
+                e.array(0)?;
+            }
+        }
+
+        Ok(())
+    }
+}