diff --git a/src/tinygp/means.py b/src/tinygp/means.py
index 9811d634..0db37b17 100644
--- a/src/tinygp/means.py
+++ b/src/tinygp/means.py
@@ -39,18 +39,18 @@ class Mean(MeanBase):
             signature.
     """
 
-    value: JAXArray | None = None
+    value: JAXArray
     func: Callable[[JAXArray], JAXArray] | None = eqx.field(default=None, static=True)
 
     def __init__(self, value: JAXArray | Callable[[JAXArray], JAXArray]):
         if callable(value):
             self.func = value
+            self.value = jax.numpy.zeros(())
         else:
             self.value = value
 
     def __call__(self, X: JAXArray) -> JAXArray:
-        if self.value is None:
-            assert self.func is not None
+        if self.func is not None:
             return self.func(X)
         return self.value
 
diff --git a/src/tinygp/test_utils.py b/src/tinygp/test_utils.py
index dd469d84..e55d211b 100644
--- a/src/tinygp/test_utils.py
+++ b/src/tinygp/test_utils.py
@@ -1,3 +1,4 @@
+from contextlib import contextmanager
 from typing import Any
 
 import jax
@@ -30,3 +31,29 @@ def assert_pytrees_allclose(calculated: Any, expected: Any, *args: Any, **kwargs
     jax.tree_util.tree_map(
         lambda a, b: assert_allclose(a, b, *args, **kwargs), calculated, expected
     )
+
+
+def _as_context_manager(obj):
+    # If it's already a context manager
+    if hasattr(obj, "__enter__") and hasattr(obj, "__exit__"):
+        return obj
+
+    # If it's a generator, wrap it
+    if hasattr(obj, "__iter__") and hasattr(obj, "send"):
+        return contextmanager(lambda: obj)()
+
+    raise TypeError("Object is neither a context manager nor a generator")
+
+
+@contextmanager
+def jax_enable_x64():
+    if hasattr(jax, "enable_x64"):
+        cm = jax.enable_x64(True)
+    else:
+        # deprecated in jax>=0.9
+        from jax.experimental import enable_x64 as _enable_x64
+
+        cm = _enable_x64()
+
+    with _as_context_manager(cm):
+        yield
diff --git a/tests/test_gp.py b/tests/test_gp.py
index 0c478e0c..be77c11b 100644
--- a/tests/test_gp.py
+++ b/tests/test_gp.py
@@ -6,7 +6,7 @@
 from numpy import random as np_random
 
 from tinygp import GaussianProcess, kernels
-from tinygp.test_utils import assert_allclose
+from tinygp.test_utils import assert_allclose, jax_enable_x64
 
 
 @pytest.fixture
@@ -24,7 +24,7 @@ def data(random):
 def test_sample(data):
     X, _ = data
 
-    with jax.experimental.enable_x64(True):
+    with jax_enable_x64():
         gp = GaussianProcess(
             kernels.Matern32(1.5), X, diag=0.01, mean=lambda x: jnp.sum(x)
         )
diff --git a/tests/test_kernels/test_kernels.py b/tests/test_kernels/test_kernels.py
index 6e05883d..f41347fc 100644
--- a/tests/test_kernels/test_kernels.py
+++ b/tests/test_kernels/test_kernels.py
@@ -5,7 +5,7 @@
 
 from tinygp import kernels, noise
 from tinygp.solvers import DirectSolver
-from tinygp.test_utils import assert_allclose
+from tinygp.test_utils import assert_allclose, jax_enable_x64
 
 
 @pytest.fixture
@@ -71,7 +71,7 @@ def test_ops(data):
 
 def test_conditioned(data):
     x1, x2 = data
-    with jax.experimental.enable_x64():  # type: ignore
+    with jax_enable_x64():  # type: ignore
         k1 = 1.5 * kernels.Matern32(2.5)
         k2 = 0.9 * kernels.ExpSineSquared(scale=1.5, gamma=0.3)
         K = k1(x1, x1) + 0.1 * jnp.eye(x1.shape[0])