diff --git a/mypy/checker.py b/mypy/checker.py
index e72bbb8aee0d..b966492315c7 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -607,11 +607,11 @@ def accept_loop(
         """
         # The outer frame accumulates the results of all iterations
         with self.binder.frame_context(can_skip=False, conditional_frame=True):
-            partials_old = len(self.partial_types[0].map)
+            partials_old = sum(len(pts.map) for pts in self.partial_types)
             while True:
                 with self.binder.frame_context(can_skip=True, break_frame=2, continue_frame=1):
                     self.accept(body)
-                partials_new = len(self.partial_types[0].map)
+                partials_new = sum(len(pts.map) for pts in self.partial_types)
                 if (partials_new == partials_old) and not self.binder.last_pop_changed:
                     break
                 partials_old = partials_new
diff --git a/test-data/unit/check-narrowing.test b/test-data/unit/check-narrowing.test
index 1bf1eca6e833..ad59af01010c 100644
--- a/test-data/unit/check-narrowing.test
+++ b/test-data/unit/check-narrowing.test
@@ -2379,4 +2379,13 @@ def g() -> None:
                 z[0] + "v"  # E: Unsupported operand types for + ("int" and "str")
             z.append(1)
 
+class A:
+    def g(self) -> None:
+        z = []  # E: Need type annotation for "z" (hint: "z: List[<type>] = ...")
+        for i in range(2):
+            while f():
+                if z:
+                    z[0] + "v"  # E: Unsupported operand types for + ("int" and "str")
+                z.append(1)
+
 [builtins fixtures/primitives.pyi]