diff --git a/model.py b/model.py
index fa43ed3..bb81dfd 100644
--- a/model.py
+++ b/model.py
@@ -324,15 +324,26 @@ def mutations_df(self):
         position = ts.sites_position[ts.mutations_site]
 
         tables = self.ts.tables
-        assert np.all(
-            tables.mutations.derived_state_offset == np.arange(ts.num_mutations + 1)
+        derived_state = tables.mutations.derived_state
+        offsets = tables.mutations.derived_state_offset
+        derived_state = np.array(
+            [
+                derived_state[s].tobytes().decode("utf-8")
+                for s in (
+                    slice(start, end) for start, end in zip(offsets[:-1], offsets[1:])
+                )
+            ]
         )
-        derived_state = tables.mutations.derived_state.view("S1").astype(str)
-
-        assert np.all(
-            tables.sites.ancestral_state_offset == np.arange(ts.num_sites + 1)
+        ancestral_state = tables.sites.ancestral_state
+        offsets = tables.sites.ancestral_state_offset
+        ancestral_state = np.array(
+            [
+                ancestral_state[s].tobytes().decode("utf-8")
+                for s in (
+                    slice(start, end) for start, end in zip(offsets[:-1], offsets[1:])
+                )
+            ]
         )
-        ancestral_state = tables.sites.ancestral_state.view("S1").astype(str)
         del tables
         inherited_state = ancestral_state[ts.mutations_site]
         mutations_with_parent = ts.mutations_parent != -1
diff --git a/tests/test_data_model.py b/tests/test_data_model.py
index 7388e18..2501bbb 100644
--- a/tests/test_data_model.py
+++ b/tests/test_data_model.py
@@ -17,6 +17,8 @@ def single_tree_example_ts():
     for j in range(6):
         tables.sites.add_row(position=j + 1, ancestral_state="A")
         tables.mutations.add_row(site=j, derived_state="T", node=j)
+    tables.sites.add_row(position=7, ancestral_state="FOOBAR")
+    tables.mutations.add_row(site=6, derived_state="FOOBARD", node=6)
     return tables.tree_sequence()
 
 
@@ -102,16 +104,16 @@ def test_single_tree_example(self):
         ts = single_tree_example_ts()
         tsm = model.TSModel(ts)
         df = tsm.mutations_df
-        assert len(df) == 6
-        nt.assert_array_equal(df.id, list(range(6)))
-        nt.assert_array_equal(df.node, list(range(6)))
-        nt.assert_array_equal(df.position, list(range(1, 7)))
-        nt.assert_array_equal(df.time, [0, 0, 0, 0, 1, 1])
-        nt.assert_array_equal(df.derived_state, ["T"] * 6)
-        nt.assert_array_equal(df.inherited_state, ["A"] * 6)
-        nt.assert_array_equal(df.num_parents, [0] * 6)
-        nt.assert_array_equal(df.num_descendants, [1] * 4 + [2] * 2)
-        nt.assert_array_equal(df.num_inheritors, [1] * 4 + [2] * 2)
+        assert len(df) == 7
+        nt.assert_array_equal(df.id, list(range(7)))
+        nt.assert_array_equal(df.node, list(range(7)))
+        nt.assert_array_equal(df.position, list(range(1, 8)))
+        nt.assert_array_equal(df.time, [0, 0, 0, 0, 1, 1, 2])
+        nt.assert_array_equal(df.derived_state, ["T"] * 6 + ["FOOBARD"])
+        nt.assert_array_equal(df.inherited_state, ["A"] * 6 + ["FOOBAR"])
+        nt.assert_array_equal(df.num_parents, [0] * 7)
+        nt.assert_array_equal(df.num_descendants, [1] * 4 + [2] * 2 + [4])
+        nt.assert_array_equal(df.num_inheritors, [1] * 4 + [2] * 2 + [4])
 
     def test_single_tree_recurrent_mutation_example(self):
         ts = single_tree_recurrent_mutation_example_ts()
@@ -162,7 +164,7 @@ def test_single_tree_example(self):
         df = tsm.nodes_df
         assert len(df) == 7
         nt.assert_array_equal(df.time, [0.0, 0.0, 0.0, 0.0, 1.0, 1.0, 2.0])
-        nt.assert_array_equal(df.num_mutations, [1, 1, 1, 1, 1, 1, 0])
+        nt.assert_array_equal(df.num_mutations, [1, 1, 1, 1, 1, 1, 1])
         nt.assert_array_equal(df.ancestors_span, [10, 10, 10, 10, 10, 10, -np.inf])
         nt.assert_array_equal(df.is_sample, [1, 1, 1, 1, 0, 0, 0])