diff --git a/sqlite_db/src/lib.rs b/sqlite_db/src/lib.rs
index 7026162..e80eb20 100644
--- a/sqlite_db/src/lib.rs
+++ b/sqlite_db/src/lib.rs
@@ -83,7 +83,7 @@ struct RunTxRow {
     end_timestamp: usize,
     block_number: u64,
     gas_used: String,
-    kind: String,
+    kind: Option<String>,
 }
 
 impl RunTxRow {
@@ -275,7 +275,7 @@ impl DbOps for SqliteDb {
                 tx.end_timestamp,
                 tx.block_number,
                 tx.gas_used.to_string(),
-                tx.kind,
+                tx.kind.to_owned().unwrap_or("NULL".to_string()),
             )
         });
         pool.execute_batch(&format!(
@@ -365,7 +365,7 @@ mod tests {
                 end_timestamp: 200,
                 block_number: 1,
                 gas_used: 100,
-                kind: "test".to_string(),
+                kind: Some("test".to_string()),
             },
             RunTx {
                 tx_hash: TxHash::from_slice(&[1u8; 32]),
@@ -373,7 +373,7 @@ mod tests {
                 end_timestamp: 300,
                 block_number: 2,
                 gas_used: 200,
-                kind: "test".to_string(),
+                kind: Some("test".to_string()),
             },
         ];
         db.insert_run_txs(run_id as u64, run_txs).unwrap();
diff --git a/src/db/mod.rs b/src/db/mod.rs
index 81dd232..92d5770 100644
--- a/src/db/mod.rs
+++ b/src/db/mod.rs
@@ -12,7 +12,7 @@ pub struct RunTx {
     pub end_timestamp: usize,
     pub block_number: u64,
     pub gas_used: u128,
-    pub kind: String,
+    pub kind: Option<String>,
 }
 
 pub trait DbOps {
diff --git a/src/spammer/mod.rs b/src/spammer/mod.rs
index d8b62f5..207f710 100644
--- a/src/spammer/mod.rs
+++ b/src/spammer/mod.rs
@@ -74,8 +74,7 @@ impl OnTxSent for LogCallback {
         let kind = extra
             .as_ref()
             .map(|e| e.get("kind").map(|k| k.to_string()))
-            .flatten()
-            .unwrap_or("".to_string());
+            .flatten();
         let handle = tokio::task::spawn(async move {
             let res = PendingTransactionBuilder::from_config(&rpc, tx_response);
             let tx_hash = res.tx_hash();
diff --git a/src/spammer/tx_actor.rs b/src/spammer/tx_actor.rs
index 351bbb1..bb12017 100644
--- a/src/spammer/tx_actor.rs
+++ b/src/spammer/tx_actor.rs
@@ -13,7 +13,7 @@ enum TxActorMessage {
     SentRunTx {
         tx_hash: TxHash,
         start_timestamp: usize,
-        kind: String,
+        kind: Option<String>,
         on_receipt: oneshot::Sender<()>,
     },
     FlushCache {
@@ -37,11 +37,11 @@ where
 pub struct PendingRunTx {
     tx_hash: TxHash,
     start_timestamp: usize,
-    kind: String,
+    kind: Option<String>,
 }
 
 impl PendingRunTx {
-    pub fn new(tx_hash: TxHash, start_timestamp: usize, kind: String) -> Self {
+    pub fn new(tx_hash: TxHash, start_timestamp: usize, kind: Option<String>) -> Self {
         Self {
             tx_hash,
             start_timestamp,
@@ -191,7 +191,7 @@ impl TxActorHandle {
         &self,
         tx_hash: TxHash,
         start_timestamp: usize,
-        kind: String,
+        kind: Option<String>,
     ) -> Result<(), Box<dyn std::error::Error>> {
         let (sender, receiver) = oneshot::channel();
         self.sender