diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2fca4ca471..312b71e26a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- Updated the autograd check for if a `PolySlab` is inside the simulation bounds to work when the slab thickness is zero.
+
 ## [2.9.0rc2] - 2025-07-17
 
 ### Added
diff --git a/tidy3d/components/geometry/polyslab.py b/tidy3d/components/geometry/polyslab.py
index 41a7a2ebf3..8478cb70ec 100644
--- a/tidy3d/components/geometry/polyslab.py
+++ b/tidy3d/components/geometry/polyslab.py
@@ -1449,11 +1449,13 @@ def _compute_derivatives(self, derivative_info: DerivativeInfo) -> AutogradField
 
         sim_min, sim_max = map(np.asarray, derivative_info.bounds_intersect)
         extents = sim_max - sim_min
-        is_2d = np.isclose(extents[self.axis], 0.0)
+        polyslab_2d = np.isclose(np.diff(self.slab_bounds), 0.0)
+        sim_2d = np.isclose(extents[self.axis], 0.0)
+        is_2d = polyslab_2d and sim_2d
 
         # early return if polyslab is not in simulation domain
         slab_min, slab_max = self.slab_bounds
-        if (slab_max <= sim_min[self.axis]) or (slab_min >= sim_max[self.axis]):
+        if (slab_max < sim_min[self.axis]) or (slab_min > sim_max[self.axis]):
             log.warning(
                 "'PolySlab' lies completely outside the simulation domain.",
                 log_once=True,