diff --git a/src/functions_framework/__init__.py b/src/functions_framework/__init__.py
index b7f38290..594551d3 100644
--- a/src/functions_framework/__init__.py
+++ b/src/functions_framework/__init__.py
@@ -174,6 +174,19 @@ def create_app(target=None, source=None, signature_type=None):
     app = flask.Flask(target, template_folder=template_folder)
     app.config["MAX_CONTENT_LENGTH"] = MAX_CONTENT_LENGTH
 
+    # 6. Handle legacy GCF Python 3.7 behavior
+    if os.environ.get("ENTRY_POINT"):
+        os.environ["FUNCTION_TRIGGER_TYPE"] = signature_type
+        os.environ["FUNCTION_NAME"] = os.environ.get("K_SERVICE", target)
+        app.make_response_original = app.make_response
+
+        def handle_none(rv):
+            if rv is None:
+                rv = "OK"
+            return app.make_response_original(rv)
+
+        app.make_response = handle_none
+
     # Extract the target function from the source file
     try:
         function = getattr(source_module, target)
diff --git a/tests/test_functions.py b/tests/test_functions.py
index c6eccb91..4698049e 100644
--- a/tests/test_functions.py
+++ b/tests/test_functions.py
@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import os
 import pathlib
 import re
 import time
@@ -455,3 +456,42 @@ def test_class_in_main_is_in_right_module():
     resp = client.get("/")
 
     assert resp.status_code == 200
+
+
+def test_function_returns_none():
+    source = TEST_FUNCTIONS_DIR / "returns_none" / "main.py"
+    target = "function"
+
+    client = create_app(target, source).test_client()
+    resp = client.get("/")
+
+    assert resp.status_code == 500
+
+
+def test_legacy_function_check_env(monkeypatch):
+    source = TEST_FUNCTIONS_DIR / "http_check_env" / "main.py"
+    target = "function"
+
+    monkeypatch.setenv("ENTRY_POINT", target)
+
+    client = create_app(target, source).test_client()
+    resp = client.post("/", json={"mode": "FUNCTION_TRIGGER_TYPE"})
+    assert resp.status_code == 200
+    assert resp.data == b"http"
+
+    resp = client.post("/", json={"mode": "FUNCTION_NAME"})
+    assert resp.status_code == 200
+    assert resp.data.decode("utf-8") == target
+
+
+def test_legacy_function_returns_none(monkeypatch):
+    source = TEST_FUNCTIONS_DIR / "returns_none" / "main.py"
+    target = "function"
+
+    monkeypatch.setenv("ENTRY_POINT", target)
+
+    client = create_app(target, source).test_client()
+    resp = client.get("/")
+
+    assert resp.status_code == 200
+    assert resp.data == b"OK"
diff --git a/tests/test_functions/returns_none/main.py b/tests/test_functions/returns_none/main.py
new file mode 100644
index 00000000..f6a4acaa
--- /dev/null
+++ b/tests/test_functions/returns_none/main.py
@@ -0,0 +1,27 @@
+# Copyright 2020 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+def function(request):
+    """Test HTTP function when using legacy GCF behavior.
+
+  The function returns None, which should be a 200 response.
+
+  Args:
+    request: The HTTP request which triggered this function.
+
+  Returns:
+    None.
+  """
+    return None