diff --git a/mypy/partially_defined.py b/mypy/partially_defined.py
index 47cbd671f168..b7f577110fa8 100644
--- a/mypy/partially_defined.py
+++ b/mypy/partially_defined.py
@@ -506,7 +506,7 @@ def visit_break_stmt(self, o: BreakStmt) -> None:
         self.tracker.skip_branch()
 
     def visit_expression_stmt(self, o: ExpressionStmt) -> None:
-        if isinstance(self.type_map.get(o.expr, None), UninhabitedType):
+        if isinstance(self.type_map.get(o.expr, None), (UninhabitedType, type(None))):
             self.tracker.skip_branch()
         super().visit_expression_stmt(o)
 
diff --git a/test-data/unit/check-possibly-undefined.test b/test-data/unit/check-possibly-undefined.test
index ebceef88b537..ae277949c049 100644
--- a/test-data/unit/check-possibly-undefined.test
+++ b/test-data/unit/check-possibly-undefined.test
@@ -1026,3 +1026,20 @@ class B:
         else:
             # Same as above but in a loop.
             b = a  # E: Name "a" may be undefined
+
+[case testUnreachableCausingMissingTypeMap]
+# flags: --enable-error-code possibly-undefined --enable-error-code used-before-def --no-warn-unreachable
+# Regression test for https://github.com/python/mypy/issues/15958
+from typing import Union, NoReturn
+
+def assert_never(__x: NoReturn) -> NoReturn: ...
+
+def foo(x: Union[int, str]) -> None:
+    if isinstance(x, str):
+        f = "foo"
+    elif isinstance(x, int):
+        f = "bar"
+    else:
+        assert_never(x)
+    f  # OK
+[builtins fixtures/tuple.pyi]