diff --git a/mypy/constraints.py b/mypy/constraints.py
index ca254026c310..aa4ce24b65df 100644
--- a/mypy/constraints.py
+++ b/mypy/constraints.py
@@ -347,29 +347,33 @@ def visit_instance(self, template: Instance) -> List[Constraint]:
                     template.type.has_base(instance.type.fullname)):
                 mapped = map_instance_to_supertype(template, instance.type)
                 tvars = mapped.type.defn.type_vars
-                for i in range(len(instance.args)):
+                # N.B: We use zip instead of indexing because the lengths might have
+                # mismatches during daemon reprocessing.
+                for tvar, mapped_arg, instance_arg in zip(tvars, mapped.args, instance.args):
                     # The constraints for generic type parameters depend on variance.
                     # Include constraints from both directions if invariant.
-                    if tvars[i].variance != CONTRAVARIANT:
+                    if tvar.variance != CONTRAVARIANT:
                         res.extend(infer_constraints(
-                            mapped.args[i], instance.args[i], self.direction))
-                    if tvars[i].variance != COVARIANT:
+                            mapped_arg, instance_arg, self.direction))
+                    if tvar.variance != COVARIANT:
                         res.extend(infer_constraints(
-                            mapped.args[i], instance.args[i], neg_op(self.direction)))
+                            mapped_arg, instance_arg, neg_op(self.direction)))
                 return res
             elif (self.direction == SUPERTYPE_OF and
                     instance.type.has_base(template.type.fullname)):
                 mapped = map_instance_to_supertype(instance, template.type)
                 tvars = template.type.defn.type_vars
-                for j in range(len(template.args)):
+                # N.B: We use zip instead of indexing because the lengths might have
+                # mismatches during daemon reprocessing.
+                for tvar, mapped_arg, template_arg in zip(tvars, mapped.args, template.args):
                     # The constraints for generic type parameters depend on variance.
                     # Include constraints from both directions if invariant.
-                    if tvars[j].variance != CONTRAVARIANT:
+                    if tvar.variance != CONTRAVARIANT:
                         res.extend(infer_constraints(
-                            template.args[j], mapped.args[j], self.direction))
-                    if tvars[j].variance != COVARIANT:
+                            template_arg, mapped_arg, self.direction))
+                    if tvar.variance != COVARIANT:
                         res.extend(infer_constraints(
-                            template.args[j], mapped.args[j], neg_op(self.direction)))
+                            template_arg, mapped_arg, neg_op(self.direction)))
                 return res
             if (template.type.is_protocol and self.direction == SUPERTYPE_OF and
                     # We avoid infinite recursion for structural subtypes by checking
diff --git a/mypy/join.py b/mypy/join.py
index 8989a596b70e..a2513bd36201 100644
--- a/mypy/join.py
+++ b/mypy/join.py
@@ -345,8 +345,10 @@ def join_instances(t: Instance, s: Instance) -> ProperType:
         if is_subtype(t, s) or is_subtype(s, t):
             # Compatible; combine type arguments.
             args = []  # type: List[Type]
-            for i in range(len(t.args)):
-                args.append(join_types(t.args[i], s.args[i]))
+            # N.B: We use zip instead of indexing because the lengths might have
+            # mismatches during daemon reprocessing.
+            for ta, sa in zip(t.args, s.args):
+                args.append(join_types(ta, sa))
             return Instance(t.type, args)
         else:
             # Incompatible; return trivial result object.
diff --git a/mypy/meet.py b/mypy/meet.py
index 608faf8f25fe..548278c154da 100644
--- a/mypy/meet.py
+++ b/mypy/meet.py
@@ -491,8 +491,10 @@ def visit_instance(self, t: Instance) -> ProperType:
                     # Combine type arguments. We could have used join below
                     # equivalently.
                     args = []  # type: List[Type]
-                    for i in range(len(t.args)):
-                        args.append(self.meet(t.args[i], si.args[i]))
+                    # N.B: We use zip instead of indexing because the lengths might have
+                    # mismatches during daemon reprocessing.
+                    for ta, sia in zip(t.args, si.args):
+                        args.append(self.meet(ta, sia))
                     return Instance(t.type, args)
                 else:
                     if state.strict_optional:
diff --git a/test-data/unit/fine-grained.test b/test-data/unit/fine-grained.test
index 11e83f560eee..2dc598661fd9 100644
--- a/test-data/unit/fine-grained.test
+++ b/test-data/unit/fine-grained.test
@@ -9288,3 +9288,92 @@ class B:
         self.x = 0
 [out]
 ==
+
+[case testGenericChange1]
+import a
+[file a.py]
+import b
+def f() -> b.C: pass
+[file b.py]
+import a
+class C: pass
+[file b.py.2]
+from typing import TypeVar, Generic, List
+import a
+
+T = TypeVar('T')
+class C(Generic[T]): pass
+
+reveal_type(a.f)
+c: C[int]
+l = a.f() if True else c
+d = a.f()
+d = c
+c = d
+
+x: List[C] = [a.f(), a.f()]
+
+[out]
+==
+b.py:7: note: Revealed type is 'def () -> b.C[Any]'
+[builtins fixtures/list.pyi]
+
+[case testGenericChange2]
+import a
+[file a.py]
+import b
+def f() -> b.C[int]: pass
+[file b.py]
+from typing import TypeVar, Generic
+import a
+T = TypeVar('T')
+class C(Generic[T]): pass
+[file b.py.2]
+from typing import List
+import a
+
+class C(): pass
+
+c: C
+l = a.f() if True else c
+d = a.f()
+d = c
+c = d
+
+x: List[C] = [a.f(), a.f()]
+
+[builtins fixtures/list.pyi]
+[out]
+==
+a.py:2: error: "C" expects no type arguments, but 1 given
+
+[case testGenericChange3]
+import a
+[file a.py]
+import b
+def f() -> b.C[int]: pass
+[file b.py]
+from typing import TypeVar, Generic
+import a
+T = TypeVar('T')
+class C(Generic[T]): pass
+[file b.py.2]
+from typing import TypeVar, Generic, List
+import a
+
+T = TypeVar('T')
+S = TypeVar('S')
+class C(Generic[S, T]): pass
+
+c: C[int, str]
+l = a.f() if True else c
+d = a.f()
+d = c
+c = d
+
+x: List[C] = [a.f(), a.f()]
+
+[out]
+==
+a.py:2: error: "C" expects 2 type arguments, but 1 given
+[builtins fixtures/list.pyi]