diff --git a/mypy/join.py b/mypy/join.py
index 23838ffad5ed..45b8bde47dc0 100644
--- a/mypy/join.py
+++ b/mypy/join.py
@@ -433,7 +433,7 @@ def visit_type_alias_type(self, t: TypeAliasType) -> ProperType:
         assert False, "This should be never called, got {}".format(t)
 
     def visit_type_guard_type(self, t: TypeGuardType) -> ProperType:
-        assert False, "This should be never called, got {}".format(t)
+        return t.type_guard.accept(self)
 
     def join(self, s: Type, t: Type) -> ProperType:
         return join_types(s, t)
diff --git a/test-data/unit/check-typeguard.test b/test-data/unit/check-typeguard.test
index fa340cb04044..56fcecece4bf 100644
--- a/test-data/unit/check-typeguard.test
+++ b/test-data/unit/check-typeguard.test
@@ -315,3 +315,17 @@ def coverage(obj: Any) -> bool:
         return True
     return False
 [builtins fixtures/classmethod.pyi]
+
+[case testTypeGuardAssignment]
+from typing_extensions import TypeGuard
+
+class A: pass
+class B(A): pass
+
+def guard(a: A) -> TypeGuard[B]:
+    pass
+
+a = A()
+if not guard(a):
+    a = A()
+[builtins fixtures/tuple.pyi]