diff --git a/src/main/java/com/bunq/sdk/http/ApiClient.java b/src/main/java/com/bunq/sdk/http/ApiClient.java
index d3b258b6..467d12ee 100644
--- a/src/main/java/com/bunq/sdk/http/ApiClient.java
+++ b/src/main/java/com/bunq/sdk/http/ApiClient.java
@@ -54,6 +54,12 @@
  */
 public class ApiClient {
 
+  /**
+   * Error constants.
+   */
+  private static final String ERROR_COULD_NOT_DETERMINE_RESPONSE_ID_HEADER =
+      "The response header \"X-Bunq-Client-Response-Id\" or \"x-bunq-client-response-id\" could not be found.";
+
   /**
    * Endpoints not requiring active session for the request to succeed.
    */
@@ -79,6 +85,8 @@ public class ApiClient {
   private static final String HEADER_GEOLOCATION = "X-Bunq-Geolocation";
   private static final String HEADER_SIGNATURE = "X-Bunq-Client-Signature";
   private static final String HEADER_AUTHENTICATION = "X-Bunq-Client-Authentication";
+  private static final String HEADER_RESPONSE_ID_LOWER_CASE = "x-bunq-client-response-id";
+  private static final String HEADER_RESPONSE_ID_UPPER_CASE = "X-Bunq-Client-Response-Id";
 
   /**
    * Field constants.