diff --git a/src/dotenv/main.py b/src/dotenv/main.py
index 93d617d6..68ee02a0 100644
--- a/src/dotenv/main.py
+++ b/src/dotenv/main.py
@@ -275,6 +275,7 @@ def _is_interactive():
             current_file = __file__
 
         while frame.f_code.co_filename == current_file:
+            assert frame.f_back is not None
             frame = frame.f_back
         frame_filename = frame.f_code.co_filename
         path = os.path.dirname(os.path.abspath(frame_filename))
diff --git a/tox.ini b/tox.ini
index 0c69795c..a5dc050b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -22,7 +22,7 @@ commands =
 skip_install = true
 deps =
   flake8
-  mypy==0.761
+  mypy
 commands =
   flake8 src tests
   mypy --python-version=3.8 src tests