diff --git a/cryptoconditions/types/threshold.py b/cryptoconditions/types/threshold.py
index 96f1838..450c89a 100644
--- a/cryptoconditions/types/threshold.py
+++ b/cryptoconditions/types/threshold.py
@@ -4,7 +4,7 @@
 from pyasn1.codec.native.decoder import decode as nat_decode
 
 from cryptoconditions.condition import Condition
-from cryptoconditions.exceptions import MissingDataError
+from cryptoconditions.exceptions import MissingDataError, ValidationError
 from cryptoconditions.fulfillment import Fulfillment
 from cryptoconditions.types.ed25519 import Ed25519Sha256
 from cryptoconditions.types.base_sha256 import BaseSha256
@@ -55,8 +55,12 @@ def __init__(self, threshold=None):
         Args:
             threshold (int): Integer threshold
         """
-        if threshold and (not isinstance(threshold, int) or threshold < 1):
-            raise ValueError('Threshold must be a integer greater than zero, was: {}'.format(threshold))
+        if threshold is not None and (
+                not isinstance(threshold, int) or threshold < 1):
+            raise ValueError(
+                'Threshold must be a integer greater than zero, was: {}'.
+                format(threshold)
+            )
         self.threshold = threshold
         self.subconditions = []
 
@@ -141,7 +145,7 @@ def asn1_dict_payload(self):
             key=lambda x: x['body'].cost,
         )
         if len(subfulfillments) < self.threshold:
-            raise Exception('Not enough fulfillments')
+            raise ValidationError('Not enough fulfillments')
 
         minimal_fulfillments = subfulfillments[:self.threshold]
         remaining_conditions = chain(
diff --git a/tests/types/test_threshold.py b/tests/types/test_threshold.py
new file mode 100644
index 0000000..8b79df5
--- /dev/null
+++ b/tests/types/test_threshold.py
@@ -0,0 +1,86 @@
+from pytest import mark, raises
+
+
+class TestThresholdSha256:
+
+    @mark.parametrize('threshold', (0, 0.1, 'a'))
+    def test_init_with_invalid_threshold(self, threshold):
+        from cryptoconditions.types.threshold import ThresholdSha256
+        with raises(ValueError) as exc:
+            ThresholdSha256(threshold=threshold)
+        assert exc.value.args == (
+            'Threshold must be a integer greater than zero, was: {}'.
+            format(threshold),
+        )
+
+    def test_add_subcondition_type_error(self):
+        from cryptoconditions.types.threshold import ThresholdSha256
+        threshold_obj = ThresholdSha256()
+        with raises(TypeError) as exc:
+            threshold_obj.add_subcondition(123)
+        assert exc.value.args == (
+            'Subconditions must be URIs or objects of type Condition',)
+
+    def test_add_subcondition_as_uri(self, minimal_threshold):
+        from cryptoconditions.types.threshold import ThresholdSha256
+        from cryptoconditions.fulfillment import Fulfillment
+        threshold_obj = ThresholdSha256(
+            threshold=minimal_threshold.json['threshold'])
+        subfulfillment = Fulfillment.from_json(
+            minimal_threshold.json['subfulfillments'][0])
+        subcondition_uri = subfulfillment.condition_uri
+        threshold_obj.add_subcondition(subcondition_uri)
+        threshold_obj.serialize_uri == minimal_threshold.fulfillment
+
+    def test_add_subcondition_as_object(self, minimal_threshold):
+        from cryptoconditions.types.threshold import ThresholdSha256
+        from cryptoconditions.fulfillment import Fulfillment
+        threshold_obj = ThresholdSha256(
+            threshold=minimal_threshold.json['threshold'])
+        subfulfillment = Fulfillment.from_json(
+            minimal_threshold.json['subfulfillments'][0])
+        subcondition_object = subfulfillment.condition
+        threshold_obj.add_subcondition(subcondition_object)
+        threshold_obj.serialize_uri == minimal_threshold.fulfillment
+
+    def test_add_subfulfillment_type_error(self):
+        from cryptoconditions.types.threshold import ThresholdSha256
+        threshold_obj = ThresholdSha256()
+        with raises(TypeError) as exc:
+            threshold_obj.add_subfulfillment(123)
+        assert exc.value.args == (
+            'Subfulfillments must be URIs or objects of type Fulfillment',)
+
+    def test_add_subfulfillment_as_uri(self, minimal_threshold):
+        from cryptoconditions.types.threshold import ThresholdSha256
+        from cryptoconditions.fulfillment import Fulfillment
+        threshold_obj = ThresholdSha256(
+            threshold=minimal_threshold.json['threshold'])
+        subfulfillment = Fulfillment.from_json(
+            minimal_threshold.json['subfulfillments'][0])
+        subfulfillment_uri = subfulfillment.serialize_uri()
+        threshold_obj.add_subfulfillment(subfulfillment_uri)
+        threshold_obj.serialize_uri == minimal_threshold.fulfillment
+
+    def test_add_subfulfillment_as_object(self, minimal_threshold):
+        from cryptoconditions.types.threshold import ThresholdSha256
+        from cryptoconditions.fulfillment import Fulfillment
+        threshold_obj = ThresholdSha256(
+            threshold=minimal_threshold.json['threshold'])
+        subfulfillment_object = Fulfillment.from_json(
+            minimal_threshold.json['subfulfillments'][0])
+        threshold_obj.add_subfulfillment(subfulfillment_object)
+        threshold_obj.serialize_uri == minimal_threshold.fulfillment
+
+    def test_asn1_dict_payload(self):
+        from cryptoconditions.exceptions import ValidationError
+        from cryptoconditions.types.threshold import ThresholdSha256
+        threshold_obj = ThresholdSha256(threshold=1)
+        with raises(ValidationError) as exc:
+            threshold_obj.asn1_dict_payload
+        assert exc.value.args == ('Not enough fulfillments',)
+
+    def test_calculate_worst_case_length(self):
+        from cryptoconditions.types.threshold import ThresholdSha256
+        cost = ThresholdSha256.calculate_worst_case_length(1, ())
+        assert cost == float('-inf')