diff --git a/selfdrive/controls/lib/longitudinal_planner.py b/selfdrive/controls/lib/longitudinal_planner.py
index 9113bb5a96..ff5f901b52 100755
--- a/selfdrive/controls/lib/longitudinal_planner.py
+++ b/selfdrive/controls/lib/longitudinal_planner.py
@@ -62,9 +62,9 @@ class LongitudinalPlanner:
 
   @staticmethod
   def parse_model(model_msg, model_error):
-    if (len(model_msg.position.x) == 33 and
-       len(model_msg.velocity.x) == 33 and
-       len(model_msg.acceleration.x) == 33):
+    if (len(model_msg.position.x) == ModelConstants.IDX_N and
+       len(model_msg.velocity.x) == ModelConstants.IDX_N and
+       len(model_msg.acceleration.x) == ModelConstants.IDX_N):
       x = np.interp(T_IDXS_MPC, ModelConstants.T_IDXS, model_msg.position.x) - model_error * T_IDXS_MPC
       v = np.interp(T_IDXS_MPC, ModelConstants.T_IDXS, model_msg.velocity.x) - model_error
       a = np.interp(T_IDXS_MPC, ModelConstants.T_IDXS, model_msg.acceleration.x)