diff --git a/tests/test_inference.py b/tests/test_inference.py
index 1e05ef1f..2ad9f59d 100644
--- a/tests/test_inference.py
+++ b/tests/test_inference.py
@@ -40,6 +40,11 @@ class TestPrebuilt:
     Tests for tsdate on prebuilt tree sequences
     """
 
+    def test_invalid_method_failure(self):
+        ts = utility_functions.two_tree_mutation_ts()
+        with pytest.raises(ValueError, match="method must be one of"):
+            tsdate.date(ts, population_size=1, mutation_rate=None, method="foo")
+
     def test_no_population_size(self):
         ts = utility_functions.two_tree_mutation_ts()
         with pytest.raises(ValueError, match="Must specify population size"):
diff --git a/tsdate/core.py b/tsdate/core.py
index 1846a445..81c30e9d 100644
--- a/tsdate/core.py
+++ b/tsdate/core.py
@@ -1194,6 +1194,14 @@ def constrain_ages_topo(ts, node_times, eps, progress=False):
     return new_node_times
 
 
+def check_method(method):
+    if method not in ["inside_outside", "maximization", "variational_gamma"]:
+        raise ValueError(
+            "method must be one of 'inside_outside', 'maximization', "
+            "'variational_gamma'"
+        )
+
+
 def date(
     tree_sequence,
     mutation_rate,
@@ -1293,6 +1301,10 @@ def date(
         from the inside algorithm.
     :rtype: tskit.TreeSequence or (tskit.TreeSequence, dict)
     """
+
+    # check valid method - raise error if unknown.
+    check_method(method)
+
     if time_units is None:
         time_units = "generations"
     if Ne is not None:
diff --git a/tsdate/prior.py b/tsdate/prior.py
index f0ad2f9d..1843dc3f 100644
--- a/tsdate/prior.py
+++ b/tsdate/prior.py
@@ -462,6 +462,12 @@ def __init__(self, tree_sequence, *, progress=False, allow_unary=False):
             lambda: defaultdict(lambda: defaultdict(base.FLOAT_DTYPE))
         )
 
+        if not allow_unary:
+            if has_locally_unary_nodes(self.ts):
+                raise ValueError(
+                    "The input tree sequence has unary nodes: tsdate currently requires that these are removed using `simplify(keep_unary=False)`"
+                )
+
         with tqdm(total=3, desc="TipCount", disable=not self.progress) as progressbar:
             (
                 node_spans,
@@ -1256,3 +1262,13 @@ def parameter_grid(
         progress,
     )
     return mixture_prior.make_parameter_grid(population_size)
+
+
+def has_locally_unary_nodes(ts):
+    for tree, ediff in zip(ts.trees(), ts.edge_diffs()):
+        changed = {
+            e.parent for edges in (ediff.edges_out, ediff.edges_in) for e in edges
+        }
+        if (tree.num_children_array[list(changed)] == 1).any():
+            return True
+    return False