diff --git a/tests/test_fit.py b/tests/test_fit.py
index 6a175be..17aa8ad 100644
--- a/tests/test_fit.py
+++ b/tests/test_fit.py
@@ -33,7 +33,7 @@
 # Too long, delete some values for quick starts, e.g. by deleting the parameters in nan_percent, size_one_percent
 deltas_test_values = [
     ("totals", "split_fits", "force_dim", "extra_dim"),  # how
-    ("lp", "lasso"),  # solver
+    ("lp", "lasso", "tree"),  # solver
     (True,),  # plot_is_static
     (explain_changes_in_average, explain_changes_in_totals),  # function
     (0.0, 90.0),  # nan_percent
@@ -44,7 +44,7 @@
 
 # possible values for explain_levels
 levels_test_values = [
-    ("lp", "lasso"),  # solver
+    ("lp", "lasso", "tree"),  # solver
     (0.0, 90.0),  # nan_percent
     (0.0, 90.0),  # size_one_percent
 ]
diff --git a/wise_pizza/solve/tree.py b/wise_pizza/solve/tree.py
index 0f37856..eda97c5 100644
--- a/wise_pizza/solve/tree.py
+++ b/wise_pizza/solve/tree.py
@@ -73,7 +73,7 @@ def __init__(
         self.fitter = fitter
         self.dims = dims
         self._best_submodels = None
-        self._error_improvement = None
+        self._error_improvement = float("-inf")
         self.children = None
         self.dim_split = dim_split or {}
         self.depth = depth