diff --git a/mypy/checker.py b/mypy/checker.py
index ae829d1157c1..5046d9431b4f 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -3915,7 +3915,7 @@ def find_isinstance_check_helper(self, node: Expression) -> Tuple[TypeMap, TypeM
 
                     # We only try and narrow away 'None' for now
                     if not is_optional(item_type):
-                        pass
+                        continue
 
                     collection_item_type = get_proper_type(builtin_item_type(collection_type))
                     if collection_item_type is None or is_optional(collection_item_type):
diff --git a/test-data/unit/check-optional.test b/test-data/unit/check-optional.test
index 9c40d550699e..15698e99ddf5 100644
--- a/test-data/unit/check-optional.test
+++ b/test-data/unit/check-optional.test
@@ -550,6 +550,15 @@ else:
     reveal_type(b)  # N: Revealed type is 'Union[__main__.A, None]'
 [builtins fixtures/ops.pyi]
 
+[case testInferInWithErasedTypes]
+from typing import TypeVar, Callable
+
+T = TypeVar('T')
+def foo(f: Callable[[T], bool], it: T) -> None: ...
+
+foo(lambda x: x in [1, 2] and bool(), 3)
+[builtins fixtures/list.pyi]
+
 [case testWarnNoReturnWorksWithStrictOptional]
 # flags: --warn-no-return
 def f() -> None: