diff --git a/HISTORY.rst b/HISTORY.rst
index 4793d3d31..bf694ca49 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -6,6 +6,8 @@ Bug tracker at https://github.com/giampaolo/psutil/issues
 **Bug fixes**
 
 - #632: [Linux] better error message if cannot parse process UNIX connections.
+- #635: [UNIX] crash on module import if 'enum' package is installed on python
+  < 3.4.
 
 
 3.0.0 - 2015-06-13
diff --git a/Makefile b/Makefile
index f309e84fb..18bc45e18 100644
--- a/Makefile
+++ b/Makefile
@@ -52,6 +52,7 @@ install-dev-deps:
 		mock \
 		nose \
 		pep8 \
+		pyflakes \
 		sphinx \
 		sphinx-pypi-upload \
 		unittest2 \
@@ -80,7 +81,7 @@ test-memleaks: install
 test-by-name: install
 	@$(PYTHON) -m nose test/test_psutil.py --nocapture -v -m $(filter-out $@,$(MAKECMDGOALS))
 
-# same as above but for test_memory_leaks.py script
+# Same as above but for test_memory_leaks.py script.
 test-memleaks-by-name: install
 	@$(PYTHON) -m nose test/test_memory_leaks.py --nocapture -v -m $(filter-out $@,$(MAKECMDGOALS))
 
@@ -92,16 +93,13 @@ coverage: install
 	$(PYTHON) -m coverage html $(COVERAGE_OPTS)
 	$(PYTHON) -m webbrowser -t htmlcov/index.html
 
-# requires "pip install pep8"
 pep8:
 	@git ls-files | grep \\.py$ | xargs $(PYTHON) -m pep8
 
-# requires "pip install pyflakes"
 pyflakes:
 	@export PYFLAKES_NODOCTEST=1 && \
 		git ls-files | grep \\.py$ | xargs $(PYTHON) -m pyflakes
 
-# requires "pip install flake8"
 flake8:
 	@git ls-files | grep \\.py$ | xargs $(PYTHON) -m flake8
 
diff --git a/psutil/_common.py b/psutil/_common.py
index 132d9d593..e9acf595d 100644
--- a/psutil/_common.py
+++ b/psutil/_common.py
@@ -12,17 +12,19 @@
 import os
 import socket
 import stat
+import sys
+from collections import namedtuple
+from socket import AF_INET, SOCK_STREAM, SOCK_DGRAM
 try:
     import threading
 except ImportError:
     import dummy_threading as threading
-try:
-    import enum  # py >= 3.4
-except ImportError:
+
+if sys.version_info >= (3, 4):
+    import enum
+else:
     enum = None
 
-from collections import namedtuple
-from socket import AF_INET, SOCK_STREAM, SOCK_DGRAM
 
 # --- constants