diff --git a/README.rst b/README.rst
index b07d74d..9b363c2 100644
--- a/README.rst
+++ b/README.rst
@@ -117,7 +117,7 @@ To make sure your cache generation scripts don't race, use the `Lock` class:
 
 >>> import portalocker
 >>> with portalocker.Lock('somefile', timeout=1) as fh:
-...     print >>fh, 'writing some stuff to my cache...'
+...     print('writing some stuff to my cache...', file=fh)
 
 To customize the opening and locking a manual approach is also possible:
 
diff --git a/portalocker/__about__.py b/portalocker/__about__.py
index 0762d9d..17bf0ed 100644
--- a/portalocker/__about__.py
+++ b/portalocker/__about__.py
@@ -1,7 +1,7 @@
 __package_name__ = 'portalocker'
 __author__ = 'Rick van Hattem'
 __email__ = 'wolph@wol.ph'
-__version__ = '2.5.0'
+__version__ = '2.5.1'
 __description__ = '''Wraps the portalocker recipe for easy usage'''
 __url__ = 'https://github.com/WoLpH/portalocker'
 
diff --git a/portalocker/__init__.py b/portalocker/__init__.py
index 03eaee9..c0edcc1 100644
--- a/portalocker/__init__.py
+++ b/portalocker/__init__.py
@@ -17,7 +17,7 @@
 #: Current author's email address
 __email__ = __about__.__email__
 #: Version number
-__version__ = '2.5.0'
+__version__ = '2.5.1'
 #: Package description for Pypi
 __description__ = __about__.__description__
 #: Package homepage
diff --git a/portalocker/portalocker.py b/portalocker/portalocker.py
index 3be4d66..4bae5e3 100644
--- a/portalocker/portalocker.py
+++ b/portalocker/portalocker.py
@@ -5,6 +5,12 @@
 from . import constants
 from . import exceptions
 
+
+# Alias for readability. Due to import recursion issues we cannot do:
+# from .constants import LockFlags
+LockFlags = constants.LockFlags
+
+
 if os.name == 'nt':  # pragma: no cover
     import msvcrt
     import pywintypes
@@ -15,12 +21,12 @@
     __overlapped = pywintypes.OVERLAPPED()
 
 
-    def lock(file_: typing.IO, flags: constants.LockFlags):
+    def lock(file_: typing.IO, flags: LockFlags):
         mode = 0
-        if flags & constants.LockFlags.NON_BLOCKING:
+        if flags & LockFlags.NON_BLOCKING:
             mode |= win32con.LOCKFILE_FAIL_IMMEDIATELY
 
-        if flags & constants.LockFlags.EXCLUSIVE:
+        if flags & LockFlags.EXCLUSIVE:
             mode |= win32con.LOCKFILE_EXCLUSIVE_LOCK
 
         # Save the old position so we can go back to that position but
@@ -85,13 +91,20 @@ def unlock(file_: typing.IO):
     import fcntl
 
 
-    def lock(file_: typing.IO, flags: constants.LockFlags):
+    def lock(file_: typing.IO, flags: LockFlags):
         locking_exceptions = IOError,
         try:  # pragma: no cover
             locking_exceptions += BlockingIOError,  # type: ignore
         except NameError:  # pragma: no cover
             pass
 
+        # Locking with NON_BLOCKING without EXCLUSIVE or SHARED enabled results
+        # in an error
+        if ((flags & LockFlags.NON_BLOCKING)
+                and not flags & (LockFlags.SHARED | LockFlags.EXCLUSIVE)):
+            raise RuntimeError('When locking in non-blocking mode the SHARED '
+                               'or EXCLUSIVE flag must be specified as well')
+
         try:
             fcntl.flock(file_.fileno(), flags)
         except locking_exceptions as exc_value:
@@ -101,7 +114,7 @@ def lock(file_: typing.IO, flags: constants.LockFlags):
 
 
     def unlock(file_: typing.IO, ):
-        fcntl.flock(file_.fileno(), constants.LockFlags.UNBLOCK)
+        fcntl.flock(file_.fileno(), LockFlags.UNBLOCK)
 
 else:  # pragma: no cover
     raise RuntimeError('PortaLocker only defined for nt and posix platforms')
diff --git a/portalocker_tests/tests.py b/portalocker_tests/tests.py
index bd3ba51..e41cb03 100644
--- a/portalocker_tests/tests.py
+++ b/portalocker_tests/tests.py
@@ -1,6 +1,7 @@
 from __future__ import print_function
 from __future__ import with_statement
 
+import os
 import dataclasses
 import multiprocessing
 import time
@@ -227,6 +228,14 @@ def test_blocking_timeout(tmpfile):
         lock.acquire(timeout=5)
 
 
+@pytest.mark.skipif(os.name == 'nt',
+                    reason='Windows uses an entirely different lockmechanism')
+def test_nonblocking(tmpfile):
+    with open(tmpfile, 'w') as fh:
+        with pytest.raises(RuntimeError):
+            portalocker.lock(fh, LockFlags.NON_BLOCKING)
+
+
 def shared_lock(filename, **kwargs):
     with portalocker.Lock(
         filename,
diff --git a/setup.cfg b/setup.cfg
index 70e8910..9f4fba1 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -6,5 +6,5 @@ universal = 1
 
 [flake8]
 ignore =
-    *.py W391,E303
+    *.py W391,E303,W503
     docs/*.py ALL