diff --git a/Src/IronPython.Modules/mmap.cs b/Src/IronPython.Modules/mmap.cs
index 942f35a7d..b6cbb871e 100644
--- a/Src/IronPython.Modules/mmap.cs
+++ b/Src/IronPython.Modules/mmap.cs
@@ -812,6 +812,10 @@ public void resize(long newsize) {
                             if (_handle.IsInvalid) {
                                 throw PythonNT.GetOsError(PythonErrno.EBADF);
                             }
+                            if (_view.Capacity == newsize) {
+                                // resizing to the same size
+                                return;
+                            }
                             if (newsize == 0) {
                                 // resizing to an empty mapped region is not allowed
                                 throw PythonNT.GetOsError(PythonErrno.EINVAL);
@@ -844,14 +848,6 @@ public void resize(long newsize) {
                         throw WindowsError(PythonExceptions._OSError.ERROR_INVALID_PARAMETER);
                     }
 
-                    if (newsize == 0) {
-                        // resizing to an empty mapped region is not allowed
-                        throw WindowsError(_offset == 0
-                            ? PythonExceptions._OSError.ERROR_ACCESS_DENIED
-                            : PythonExceptions._OSError.ERROR_FILE_INVALID
-                        );
-                    }
-
                     if (_view.Capacity == newsize) {
                         // resizing to the same size
                         return;
@@ -860,6 +856,14 @@ public void resize(long newsize) {
                     long capacity = checked(_offset + newsize);
 
                     try {
+                        if (newsize == 0) {
+                            // resizing to an empty mapped region is not allowed
+                            throw WindowsError(_offset != 0 && RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
+                                ? PythonExceptions._OSError.ERROR_ACCESS_DENIED
+                                : PythonExceptions._OSError.ERROR_FILE_INVALID
+                            );
+                        }
+
                         _view.Flush();
                         _view.Dispose();
                         _file.Dispose();
@@ -1143,13 +1147,13 @@ private void EnsureOpen() {
                 }
             }
 
-            private struct MmapLocker : IDisposable {
+            private readonly struct MmapLocker : IDisposable {
                 private readonly MmapDefault _mmap;
 
                 public MmapLocker(MmapDefault mmap) {
                     _mmap = mmap;
-                    Interlocked.Increment(ref _mmap._refCount);
                     _mmap.EnsureOpen();
+                    Interlocked.Increment(ref _mmap._refCount);
                 }
 
                 #region IDisposable Members
diff --git a/Tests/modules/io_related/test_mmap.py b/Tests/modules/io_related/test_mmap.py
new file mode 100644
index 000000000..249c7a955
--- /dev/null
+++ b/Tests/modules/io_related/test_mmap.py
@@ -0,0 +1,105 @@
+# Licensed to the .NET Foundation under one or more agreements.
+# The .NET Foundation licenses this file to you under the Apache 2.0 License.
+# See the LICENSE file in the project root for more information.
+
+'''
+Tests the _mmap standard module.
+'''
+
+import sys
+import os
+import errno
+import mmap
+
+from iptest import IronPythonTestCase, is_cli, is_posix, is_windows, run_test
+
+class MmapTest(IronPythonTestCase):
+
+    def setUp(self):
+        super(MmapTest, self).setUp()
+
+        self.temp_file = os.path.join(self.temporary_dir, "temp_mmap_%d.dat" % os.getpid())
+
+    def tearDown(self):
+        self.delete_files(self.temp_file)
+        return super().tearDown()
+
+
+    def test_constants(self):
+        self.assertTrue(hasattr(mmap, "PAGESIZE"))
+        self.assertTrue(hasattr(mmap, "ALLOCATIONGRANULARITY"))
+
+        self.assertEqual(mmap.ACCESS_READ, 1)
+        self.assertEqual(mmap.ACCESS_WRITE, 2)
+        self.assertEqual(mmap.ACCESS_COPY, 3)
+        if sys.version_info >= (3, 7) or is_cli:
+            self.assertEqual(mmap.ACCESS_DEFAULT, 0)
+        self.assertFalse(hasattr(mmap, "ACCESS_NONE"))
+
+        if is_posix:
+            self.assertEqual(mmap.MAP_SHARED, 1)
+            self.assertEqual(mmap.MAP_PRIVATE, 2)
+            self.assertEqual(mmap.PROT_READ, 1)
+            self.assertEqual(mmap.PROT_WRITE, 2)
+            self.assertEqual(mmap.PROT_EXEC, 4)
+
+
+    def test_resize_errors(self):
+        with open(self.temp_file, "wb+") as f:
+            f.write(b"x" * mmap.ALLOCATIONGRANULARITY * 2)
+
+        with open(self.temp_file, "rb+") as f:
+            m = mmap.mmap(f.fileno(), 0, offset=0)
+            with self.assertRaises(OSError) as cm:
+                m.resize(0)
+
+            self.assertEqual(cm.exception.errno, errno.EINVAL)  # 22
+            if is_windows:
+                self.assertEqual(cm.exception.winerror, 1006)  #  ERROR_FILE_INVALID
+                self.assertEqual(cm.exception.strerror, "The volume for a file has been externally altered so that the opened file is no longer valid")
+            else:
+                self.assertEqual(cm.exception.strerror, "Invalid argument")
+
+            self.assertTrue(m.closed)
+
+
+    def test_resize_errors_negative(self):
+        with open(self.temp_file, "wb+") as f:
+            f.write(b"x" * mmap.ALLOCATIONGRANULARITY * 2)
+
+        with open(self.temp_file, "rb+") as f:
+            m = mmap.mmap(f.fileno(), 0, offset=0)
+            if is_cli or sys.version_info >= (3, 5):
+                self.assertRaises(ValueError, m.resize, -1)
+                self.assertFalse(m.closed)
+            else:
+                self.assertRaises(OSError, m.resize, -1)
+                self.assertTrue(m.closed)
+
+            m.close()
+
+
+    def test_resize_errors_offset(self):
+        with open(self.temp_file, "wb+") as f:
+            f.write(b"x" * mmap.ALLOCATIONGRANULARITY * 2)
+
+        with open(self.temp_file, "rb+") as f:
+            m = mmap.mmap(f.fileno(), 0, offset=mmap.ALLOCATIONGRANULARITY)
+
+            if is_windows:
+                with self.assertRaises(PermissionError) as cm:
+                    m.resize(0)
+                self.assertEqual(cm.exception.errno, errno.EACCES)  # 13
+                self.assertEqual(cm.exception.winerror, 5)  # ERROR_ACCESS_DENIED
+                self.assertEqual(cm.exception.strerror, "Access is denied")
+            else:
+                with self.assertRaises(OSError) as cm:
+                    m.resize(0)
+                self.assertEqual(cm.exception.errno, errno.EINVAL)  # 22
+                self.assertEqual(cm.exception.strerror, "Invalid argument")
+
+            self.assertTrue(m.closed)
+
+
+run_test(__name__)
+