diff --git a/README.md b/README.md
index d5b8a31..1d96790 100644
--- a/README.md
+++ b/README.md
@@ -42,7 +42,7 @@ Example response:
 ```
 
 Notes:
-
+- `chainId` must be the same as the `chainId` of the `FORK_URL` environment variable, if defined. 
 - `blockNumber` can be omitted and the latest block will be used, however providing a `blockNumber` is recommended where possible to use the cache.
 
 ### POST /api/v1/simulate-bundle
@@ -82,7 +82,7 @@ Example response:
 
 Notes:
 
-- `chainId` must be the same in all transactions.
+- `chainId` must be the same in all transactions and it must match the `chainId` of `FORK_URL` environment variable, if defined. 
 - `blockNumber` can be included and incremented when a multi-block simulation is required, or omitted in all transactions to use latest.
 
 ### POST /api/v1/simulate-stateful
@@ -148,7 +148,7 @@ Example response:
 ```
 Notes:
 
-- `chainId` must be the same in all transactions.
+- `chainId` must be the same in all transactions and it must match the `chainId` of `FORK_URL` environment variable, if defined.
 - `blockNumber` can be included and incremented when a multi-block simulation is required, or omitted in all transactions to use latest.
 
 
diff --git a/src/simulation.rs b/src/simulation.rs
index 1595d5c..642fdfa 100644
--- a/src/simulation.rs
+++ b/src/simulation.rs
@@ -227,9 +227,11 @@ async fn run(
 }
 
 pub async fn simulate(transaction: SimulationRequest, config: Config) -> Result<Json, Rejection> {
-    let fork_url = config
-        .fork_url
-        .unwrap_or(chain_id_to_fork_url(transaction.chain_id)?);
+    let fork_url = if let Some(url) = config.fork_url {
+        url
+    } else {
+        chain_id_to_fork_url(transaction.chain_id)?
+    };
     let mut evm = Evm::new(
         None,
         fork_url,
@@ -262,9 +264,11 @@ pub async fn simulate_bundle(
     let first_block_number = transactions[0].block_number;
     let first_block_timestamp = transactions[0].block_timestamp;
 
-    let fork_url = config
-        .fork_url
-        .unwrap_or(chain_id_to_fork_url(first_chain_id)?);
+    let fork_url = if let Some(url) = config.fork_url {
+        url
+    } else {
+        chain_id_to_fork_url(first_chain_id)?
+    };
     let mut evm = Evm::new(
         None,
         fork_url,
@@ -315,9 +319,11 @@ pub async fn simulate_stateful_new(
     config: Config,
     state: Arc<SharedSimulationState>,
 ) -> Result<Json, Rejection> {
-    let fork_url = config
-        .fork_url
-        .unwrap_or(chain_id_to_fork_url(stateful_simulation_request.chain_id)?);
+    let fork_url = if let Some(url) = config.fork_url {
+        url
+    } else {
+        chain_id_to_fork_url(stateful_simulation_request.chain_id)?
+    };
     let mut evm = Evm::new(
         None,
         fork_url,