diff --git a/.gitignore b/.gitignore
index 47d6da7..5591030 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,3 +10,7 @@ __pycache__
 .idea
 build
 dist
+
+# uv
+.venv/
+uv.lock
diff --git a/opti/problem.py b/opti/problem.py
index 63433fb..c9b5b97 100644
--- a/opti/problem.py
+++ b/opti/problem.py
@@ -1,5 +1,6 @@
 import json
 import os
+from io import StringIO
 from typing import Callable, Dict, List, Optional, Tuple, Union
 
 import numpy as np
@@ -87,10 +88,10 @@ def __init__(
             self.f = f
 
         if isinstance(data, dict):
-            data = pd.read_json(json.dumps(data), orient="split")
+            data = pd.read_json(StringIO(json.dumps(data)), orient="split")
 
         if isinstance(optima, dict):
-            optima = pd.read_json(json.dumps(optima), orient="split")
+            optima = pd.read_json(StringIO(json.dumps(optima)), orient="split")
 
         self.set_data(data)
         self.set_optima(optima)
diff --git a/opti/sampling/polytope.py b/opti/sampling/polytope.py
index 6a0354e..5edf70a 100644
--- a/opti/sampling/polytope.py
+++ b/opti/sampling/polytope.py
@@ -116,7 +116,7 @@ def _get_AbNx(parameters, constraints):
     # add the inequality constraints corresponding to the box bounds
     lower = parameters.bounds.loc["min"]
     upper = parameters.bounds.loc["max"]
-    A_ineq = np.row_stack([-np.eye(nD), np.eye(nD)])
+    A_ineq = np.vstack([-np.eye(nD), np.eye(nD)])
     b_ineq = np.r_[-np.array(lower), np.array(upper)]
 
     if sum(~is_eq) > 0:
diff --git a/test/test_sampling.py b/test/test_sampling.py
index 3951406..a4e8d59 100644
--- a/test/test_sampling.py
+++ b/test/test_sampling.py
@@ -46,7 +46,7 @@ def test_sphere_sampling():
 
 
 def test_polytope_chebyshev_center():
-    A = np.row_stack([-np.eye(3), np.eye(3)])
+    A = np.vstack([-np.eye(3), np.eye(3)])
     b = np.r_[-np.zeros(3), np.ones(3)]
     x0 = polytope._chebyshev_center(A, b)
     assert np.allclose(x0, [0.5, 0.5, 0.5])