diff --git a/mypy/build.py b/mypy/build.py
index ff9b48d2d7b4..4745610d7920 100644
--- a/mypy/build.py
+++ b/mypy/build.py
@@ -2140,6 +2140,7 @@ def parse_file(self, *, temporary: bool = False) -> None:
                     # other systems, but os.strerror(ioerr.errno) does not, so we use that.
                     # (We want the error messages to be platform-independent so that the
                     # tests have predictable output.)
+                    assert ioerr.errno is not None
                     raise CompileError(
                         [
                             "mypy: can't read file '{}': {}".format(
diff --git a/mypy/modulefinder.py b/mypy/modulefinder.py
index 9b15f2aff90e..bc11c1304776 100644
--- a/mypy/modulefinder.py
+++ b/mypy/modulefinder.py
@@ -777,6 +777,7 @@ def get_search_dirs(python_executable: str | None) -> tuple[list[str], list[str]
             print(err.stdout)
             raise
         except OSError as err:
+            assert err.errno is not None
             reason = os.strerror(err.errno)
             raise CompileError(
                 [f"mypy: Invalid python executable '{python_executable}': {reason}"]