diff --git a/pytential/qbx/__init__.py b/pytential/qbx/__init__.py
index c3ca7d95fc58877182743fb6760b1de04b65d94a..82ef3e190a96942c22ec1baa2ecf31ee319a7bcc 100644
--- a/pytential/qbx/__init__.py
+++ b/pytential/qbx/__init__.py
@@ -409,12 +409,9 @@ class QBXLayerPotentialSource(LayerPotentialSourceBase):
     @memoize_method
     def _expansion_radii(self, last_dim_length):
         if last_dim_length == "npanels":
-            # FIXME: Make this an error
-
-            from warnings import warn
-            warn("Passing 'npanels' as last_dim_length to _expansion_radii is "
-                    "deprecated. Expansion radii should be allowed to vary "
-                    "within a panel.", stacklevel=3)
+            raise ValueError(
+                    "Passing 'npanels' as last_dim_length to _expansion_radii is "
+                    "not allowed. Allowed values are 'nsources' and 'ncenters'.")
 
         with cl.CommandQueue(self.cl_context) as queue:
                 return (self._panel_sizes(last_dim_length).with_queue(queue) * 0.5
diff --git a/test/test_global_qbx.py b/test/test_global_qbx.py
index cc0f26f0ebbce0cee1ab5c423fb92f6ed94b9c66..c7609387b8bbdef3c5e74edcdd78e1bd9a2351a2 100644
--- a/test/test_global_qbx.py
+++ b/test/test_global_qbx.py
@@ -121,7 +121,7 @@ def run_source_refinement_test(ctx_getter, mesh, order, helmholtz_k=None):
     int_centers = np.array([axis.get(queue) for axis in int_centers])
     ext_centers = get_centers_on_side(lpot_source, +1)
     ext_centers = np.array([axis.get(queue) for axis in ext_centers])
-    expansion_radii = lpot_source._expansion_radii("npanels").get(queue)
+    expansion_radii = lpot_source._expansion_radii("nsources").get(queue)
     panel_sizes = lpot_source._panel_sizes("npanels").get(queue)
     fine_panel_sizes = lpot_source._fine_panel_sizes("npanels").get(queue)
 
@@ -150,8 +150,8 @@ def run_source_refinement_test(ctx_getter, mesh, order, helmholtz_k=None):
         # A center cannot be closer to another panel than to its originating
         # panel.
 
-        rad = expansion_radii[centers_panel.element_nr]
-        assert dist >= rad * (1-expansion_disturbance_tolerance), \
+        rad = expansion_radii[centers_panel.discr_slice]
+        assert (dist >= rad * (1-expansion_disturbance_tolerance)).all(), \
                 (dist, rad, centers_panel.element_nr, sources_panel.element_nr)
 
     def check_sufficient_quadrature_resolution(centers_panel, sources_panel):