diff --git a/include/bal_errors.h b/include/bal_errors.h
index 18e87427..21ddbff0 100644
--- a/include/bal_errors.h
+++ b/include/bal_errors.h
@@ -36,6 +36,8 @@ extern "C"
         /// resources.
         BAL_ERROR_THREAD_CLEANUP = -8,
 
+        BAL_ERROR_ENGINE_ALREADY_RUNNING = -9,
+
         // IR Errors.
         //
         BAL_ERROR_INSTRUCTION_OVERFLOW = -100,
diff --git a/src/bal_errors.c b/src/bal_errors.c
index fbff6a50..d8f55ae6 100644
--- a/src/bal_errors.c
+++ b/src/bal_errors.c
@@ -27,6 +27,9 @@ bal_error_to_string(const bal_error_t error)
         case BAL_ERROR_THREAD_CLEANUP:
             string = "could not clean up thread";
             break;
+        case BAL_ERROR_ENGINE_ALREADY_RUNNING:
+            string = "engine already running";
+            break;
         case BAL_ERROR_UNKNOWN_INSTRUCTION:
             string = "failed to decode arm instruction";
             break;