diff --git a/src/material.cpp b/src/material.cpp
index ec76eb988..5667821fb 100644
--- a/src/material.cpp
+++ b/src/material.cpp
@@ -1169,7 +1169,7 @@ openmc_material_add_nuclide(int32_t index, const char* name, double density)
       // Create copy of atom_density_ array with one extra entry
       xt::xtensor<double, 1> atom_density = xt::zeros<double>({n});
       xt::view(atom_density, xt::range(0, n-1)) = m->atom_density_;
-      atom_density(n) = density;
+      atom_density(n-1) = density;
       m->atom_density_ = atom_density;
 
       m->density_ += density;
diff --git a/tests/unit_tests/test_capi.py b/tests/unit_tests/test_capi.py
index 17219c82e..53ad09e90 100644
--- a/tests/unit_tests/test_capi.py
+++ b/tests/unit_tests/test_capi.py
@@ -124,6 +124,13 @@ def test_material(capi_init):
     assert m.density == pytest.approx(0.1)
 
 
+def test_material_add_nuclide(capi_init):
+    m = openmc.capi.materials[3]
+    m.add_nuclide('Xe135', 1e-12)
+    assert m.nuclides[-1] == 'Xe135'
+    assert m.densities[-1] == 1e-12
+
+
 def test_new_material(capi_init):
     with pytest.raises(exc.AllocationError):
         openmc.capi.Material(1)
@@ -135,7 +142,7 @@ def test_new_material(capi_init):
 def test_nuclide_mapping(capi_init):
     nucs = openmc.capi.nuclides
     assert isinstance(nucs, Mapping)
-    assert len(nucs) == 12
+    assert len(nucs) == 13
     for name, nuc in nucs.items():
         assert isinstance(nuc, openmc.capi.Nuclide)
         assert name == nuc.name