diff --git a/mjx/mujoco/mjx/_src/collision_driver.py b/mjx/mujoco/mjx/_src/collision_driver.py
index b5f58130..dde3fe92 100644
--- a/mjx/mujoco/mjx/_src/collision_driver.py
+++ b/mjx/mujoco/mjx/_src/collision_driver.py
@@ -336,16 +336,28 @@ def collision_candidates(m: Union[Model, mujoco.MjModel]) -> CandidateSet:
 
 def ncon(m: Model) -> int:
   """Returns the number of contacts computed in MJX given a model."""
+  if m.opt.disableflags & DisableBit.CONTACT:
+    return 0
+
   candidates = collision_candidates(m)
   max_count = _max_contact_points(m)
-  count = sum([
-      len(v) * get_collision_fn(k[0:2]).ncon for k, v in candidates.items()  # pytype: disable=attribute-error
-  ])
+
+  count = 0
+  for k, v in candidates.items():
+    fn = get_collision_fn(k[0:2])
+    if fn is None:
+      continue
+    count += len(v) * fn.ncon  # pytype: disable=attribute-error
+
   return min(max_count, count) if max_count > -1 else count
 
 
 def collision(m: Model, d: Data) -> Data:
   """Collides geometries."""
+  ncon_ = ncon(m)
+  if ncon_ == 0:
+    return d.replace(contact=Contact.zero(), ncon=0)
+
   candidate_set = collision_candidates(m)
 
   contacts = []
@@ -354,7 +366,7 @@ def collision(m: Model, d: Data) -> Data:
     contacts.append(_collide_geoms(m, d, geom_types, candidates))
 
   if not contacts:
-    return d.replace(contact=Contact.zero(), ncon=0)
+    raise RuntimeError('No contacts found.')
 
   contact = jax.tree_map(lambda *x: jp.concatenate(x), *contacts)
 
@@ -364,10 +376,13 @@ def collision(m: Model, d: Data) -> Data:
     _, idx = jax.lax.top_k(-contact.dist, k=max_contact_points)
     contact = jax.tree_map(lambda x, idx=idx: jp.take(x, idx, axis=0), contact)
 
-  ncon_ = contact.dist.shape[0]
+  if ncon_ != contact.dist.shape[0]:
+    raise RuntimeError('Number of contacts does not match ncon.')
+
+  # TODO(robotics-simulation): move this logic to device_put
   ns = d.ne + d.nf + d.nl
+  contact = contact.replace(efc_address=np.arange(ns, ns + ncon_ * 4, 4))
   # TODO(robotics-simulation): add support for other friction dimensions
-  contact = contact.replace(efc_address=np.arange(ns, ns + d.ncon * 4, 4))
   contact = contact.replace(dim=3 * np.ones(ncon_, dtype=np.int32))
 
   return d.replace(contact=contact, ncon=ncon_)
diff --git a/mjx/mujoco/mjx/_src/collision_driver_test.py b/mjx/mujoco/mjx/_src/collision_driver_test.py
index e31b95c8..b735e014 100644
--- a/mjx/mujoco/mjx/_src/collision_driver_test.py
+++ b/mjx/mujoco/mjx/_src/collision_driver_test.py
@@ -24,9 +24,12 @@ import jax
 import jax.numpy as jp
 import mujoco
 from mujoco import mjx
+from mujoco.mjx._src import collision_driver
+from mujoco.mjx._src import test_util
 # pylint: disable=g-importing-member
 from mujoco.mjx._src.types import Contact
 from mujoco.mjx._src.types import Data
+from mujoco.mjx._src.types import DisableBit
 from mujoco.mjx._src.types import Model
 # pylint: enable=g-importing-member
 import numpy as np
@@ -450,6 +453,29 @@ class BodyPairFilterTest(absltest.TestCase):
     self.assertEqual(dx.contact.pos.shape[0], 1)
 
 
+class NconTest(parameterized.TestCase):
+  """Tests ncon."""
+
+  def test_ncon(self):
+    m = test_util.load_test_file('ant.xml')
+    d = mujoco.MjData(m)
+    d.qpos[2] = 0.0
+
+    mx = mjx.device_put(m)
+    ncon = collision_driver.ncon(mx)
+    self.assertEqual(ncon, 4)
+
+  def test_disable_contact(self):
+    m = test_util.load_test_file('ant.xml')
+    d = mujoco.MjData(m)
+    d.qpos[2] = 0.0
+
+    m.opt.disableflags = m.opt.disableflags | DisableBit.CONTACT
+    mx = mjx.device_put(m)
+    ncon = collision_driver.ncon(mx)
+    self.assertEqual(ncon, 0)
+
+
 class TopKContactTest(absltest.TestCase):
   """Tests top-k contacts."""