diff --git a/graphql/error/tests/test_base.py b/graphql/error/tests/test_base.py
index 3ddc091c..fa695282 100644
--- a/graphql/error/tests/test_base.py
+++ b/graphql/error/tests/test_base.py
@@ -89,8 +89,6 @@ def resolver(context, *_):
     formatted_tb = [row[2:] for row in extracted]
     formatted_tb = [tb for tb in formatted_tb if tb[0] != "reraise"]
 
-    print(formatted_tb)
-
     assert formatted_tb == [
         ("test_reraise_from_promise", "result.errors[0].reraise()"),
         ("_resolve_from_executor", "executor(resolve, reject)"),
diff --git a/graphql/execution/executor.py b/graphql/execution/executor.py
index 47dff2d3..472f95b9 100644
--- a/graphql/execution/executor.py
+++ b/graphql/execution/executor.py
@@ -150,7 +150,7 @@ def on_resolve(data):
         return promise.get()
     else:
         clean = getattr(exe_context.executor, "clean", None)
-        if clean:
+        if callable(clean):
             clean()
 
     return promise
@@ -161,7 +161,7 @@ def execute_operation(
     operation,  # type: OperationDefinition
     root_value,  # type: Any
 ):
-    # type: (...) -> Union[Dict, Promise[Dict]]
+    # type: (...) -> Union[Dict, Promise[Dict], Observable]
     type = get_operation_root_type(exe_context.schema, operation)
     fields = collect_fields(
         exe_context, type, operation.selection_set, DefaultOrderedDict(list), set()
@@ -278,11 +278,11 @@ def on_error(error):
     def map_result(data):
         # type: (Dict[str, Any]) -> ExecutionResult
         if subscriber_exe_context.errors:
-            result = ExecutionResult(data=data, errors=subscriber_exe_context.errors)
+            res = ExecutionResult(data=data, errors=subscriber_exe_context.errors)
         else:
-            result = ExecutionResult(data=data)
+            res = ExecutionResult(data=data)
         subscriber_exe_context.reset()
-        return result
+        return res
 
     def catch_error(error):
         subscriber_exe_context.errors.append(error)
diff --git a/setup.py b/setup.py
index c8cd2311..73730ec2 100644
--- a/setup.py
+++ b/setup.py
@@ -27,8 +27,8 @@
     "pytest==4.6.9",
     "pytest-django==3.8.0",
     "pytest-cov==2.8.1",
-    "coveralls==1.10.0",
-    "gevent==1.4.0",
+    "coveralls==1.11.1",
+    "gevent>=1.4.0,<1.6",
     "six>=1.10.0",
     "pytest-benchmark==3.2.3",
     "pytest-mock==2.0.0",
diff --git a/tox.ini b/tox.ini
index 8b2bfc02..afe8751e 100644
--- a/tox.ini
+++ b/tox.ini
@@ -2,6 +2,7 @@
 envlist = py{27,35,36,37,py38,py,py3},pre-commit,mypy,docs
 
 [testenv]
+install_command = python -m pip install --ignore-installed --pre {opts} {packages}
 deps =
     .[test]
 commands =