diff --git a/src/client/transactions.rs b/src/client/transactions.rs
index 8fcf4c9..1b92a8a 100644
--- a/src/client/transactions.rs
+++ b/src/client/transactions.rs
@@ -54,11 +54,11 @@ impl Maestro {
         params: Option<HashMap<String, String>>,
     ) -> Result<TransactionOutputFromReference, Box<dyn Error>> {
         let formatted_params = params.map_or("".to_string(), |p| {
-            p.iter()
+            "?".to_string() + p.iter()
                 .map(|(k, v)| format!("{}={}", k, v))
                 .collect::<Vec<String>>()
                 .join("&")
-                .to_string()
+                .as_str()
         });
         let url = format!(
             "/transactions/{}/outputs/{}/txo{}",
diff --git a/src/models/addresses.rs b/src/models/addresses.rs
index 755c35e..56d2f7d 100644
--- a/src/models/addresses.rs
+++ b/src/models/addresses.rs
@@ -85,9 +85,9 @@ pub struct Utxo {
     pub assets: Vec<Asset>,
     pub datum: Option<HashMap<String, serde_json::Value>>,
     pub index: i64,
-    pub reference_script: ReferenceScript,
+    pub reference_script: Option<ReferenceScript>,
     pub tx_hash: String,
-    pub slot: i64,
+    #[serde(alias="txout_cbor")]
     pub tx_out_cbor: String,
 }
 
@@ -95,7 +95,7 @@ pub struct Utxo {
 pub struct ReferenceScript {
     pub bytes: String,
     pub hash: String,
-    pub json: HashMap<String, serde_json::Value>,
+    pub json: Option<HashMap<String, serde_json::Value>>,
     pub r#type: String,
 }