diff --git a/mypy/semanal.py b/mypy/semanal.py
index c5b460ebb31b..14630d964b29 100644
--- a/mypy/semanal.py
+++ b/mypy/semanal.py
@@ -4020,8 +4020,14 @@ def infer_reachability_of_if_statement(s: IfStmt,
                 mark_block_mypy_only(s.body[i])
             for body in s.body[i + 1:]:
                 mark_block_unreachable(body)
-            if s.else_body:
-                mark_block_unreachable(s.else_body)
+
+            # Make sure else body always exists and is marked as
+            # unreachable so the type checker always knows that
+            # all control flow paths will flow through the if
+            # statement body.
+            if not s.else_body:
+                s.else_body = Block([])
+            mark_block_unreachable(s.else_body)
             break
 
 
diff --git a/test-data/unit/check-unreachable-code.test b/test-data/unit/check-unreachable-code.test
index fa29290dbbf4..acf975b19193 100644
--- a/test-data/unit/check-unreachable-code.test
+++ b/test-data/unit/check-unreachable-code.test
@@ -517,3 +517,20 @@ else:
     y = 3
 reveal_type(y)  # E: Revealed type is 'builtins.str'
 [builtins fixtures/ops.pyi]
+
+[case testConditionalAssertWithoutElse]
+import typing
+
+class A: pass
+class B(A): pass
+
+x = A()
+reveal_type(x)  # E: Revealed type is '__main__.A'
+
+if typing.TYPE_CHECKING:
+    assert isinstance(x, B)
+    reveal_type(x)  # E: Revealed type is '__main__.B'
+
+reveal_type(x)  # E: Revealed type is '__main__.B'
+
+[builtins fixtures/isinstancelist.pyi]