diff --git a/pre_commit/jsonschema_extensions.py b/pre_commit/jsonschema_extensions.py
index 1187415e..d287bcaf 100644
--- a/pre_commit/jsonschema_extensions.py
+++ b/pre_commit/jsonschema_extensions.py
@@ -8,11 +8,8 @@ def extend_validator_cls(validator_cls, modify):
     validate_properties = validator_cls.VALIDATORS['properties']
 
     def new_properties(validator, properties, instance, schema):
-        for error in validate_properties(
-            validator, properties, instance, schema,
-        ):
-            yield error
-
+        # Exhaust the validator
+        list(validate_properties(validator, properties, instance, schema))
         modify(properties, instance)
 
     return jsonschema.validators.extend(
diff --git a/pre_commit/prefixed_command_runner.py b/pre_commit/prefixed_command_runner.py
index 9649648c..d8a562a2 100644
--- a/pre_commit/prefixed_command_runner.py
+++ b/pre_commit/prefixed_command_runner.py
@@ -66,9 +66,9 @@ class PrefixedCommandRunner(object):
         proc = self.__popen(replaced_cmd, **popen_kwargs)
         stdout, stderr = proc.communicate(stdin)
         # TODO: stdout, stderr = from_bytes(stdout), from_bytes(stderr)
-        if stdout is not None and not isinstance(stdout, five.text):
+        if isinstance(stdout, bytes):
             stdout = five.text(stdout, 'utf-8')
-        if stderr is not None and not isinstance(stderr, five.text):
+        if isinstance(stderr, bytes):
             stderr = five.text(stderr, 'utf-8')
         returncode = proc.returncode
 
diff --git a/tests/jsonschema_extensions_test.py b/tests/jsonschema_extensions_test.py
index 5680a333..d5a3b4d2 100644
--- a/tests/jsonschema_extensions_test.py
+++ b/tests/jsonschema_extensions_test.py
@@ -1,3 +1,6 @@
+import jsonschema.exceptions
+import pytest
+
 from pre_commit.jsonschema_extensions import apply_defaults
 from pre_commit.jsonschema_extensions import remove_defaults
 
@@ -78,3 +81,9 @@ def test_remove_defaults_removes_default():
         {'properties': {'foo': {'default': 'bar'}}},
     )
     assert ret == {}
+
+
+@pytest.mark.parametrize('func', (apply_defaults, remove_defaults))
+def test_still_validates_schema(func):
+    with pytest.raises(jsonschema.exceptions.ValidationError):
+        func({}, {'properties': {'foo': {}}, 'required': ['foo']})
diff --git a/tests/prefixed_command_runner_test.py b/tests/prefixed_command_runner_test.py
index d6b1f186..cd8523a4 100644
--- a/tests/prefixed_command_runner_test.py
+++ b/tests/prefixed_command_runner_test.py
@@ -21,7 +21,7 @@ def test_CalledProcessError_str():
 @pytest.fixture
 def popen_mock():
     popen = mock.Mock(spec=subprocess.Popen)
-    popen.return_value.communicate.return_value = ('stdout', 'stderr')
+    popen.return_value.communicate.return_value = (b'stdout', b'stderr')
     return popen