diff --git a/modepy/modal_decay.py b/modepy/modal_decay.py
index dee45fee15614f4c461be570f4d9c38b4b3eeb11..8eeb18b5af374a1ef23846d5d86f397eff5ab730 100644
--- a/modepy/modal_decay.py
+++ b/modepy/modal_decay.py
@@ -67,7 +67,10 @@ def simplex_interp_error_coefficient_estimator_matrix(
     vdm = vandermonde(basis, unit_nodes)
     vdm_inv = la.inv(vdm)
 
-    order_vector = np.array([sum(mode_id) for mode_id in mode_ids])
+    if dim > 1:
+        order_vector = np.array([sum(mode_id) for mode_id in mode_ids])
+    else:
+        order_vector = mode_ids
 
     max_order = np.max(order_vector)
     assert max_order == order