diff --git a/mesmer/stats/_harmonic_model.py b/mesmer/stats/_harmonic_model.py
index ed41097c..42d6fc8f 100644
--- a/mesmer/stats/_harmonic_model.py
+++ b/mesmer/stats/_harmonic_model.py
@@ -143,6 +143,7 @@ def func(coeffs, yearly_predictor, mon_target):
         first_guess,
         args=(yearly_predictor, monthly_target),
         loss="linear",
+        tr_solver="exact",
     )
 
     coeffs = minimize_result.x