diff --git a/.travis.yml b/.travis.yml
index b5e9e256..5bc84b59 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,7 +2,6 @@ language: python
 sudo: false   # Since this is an older project, this is not the default.
 cache: pip
 python:
-  - "2.6"
   - "2.7"
   - "3.4"
   - "3.5"
diff --git a/cybox/common/hashes.py b/cybox/common/hashes.py
index f4b89bc7..91487d8b 100644
--- a/cybox/common/hashes.py
+++ b/cybox/common/hashes.py
@@ -23,7 +23,7 @@ def _set_hash_type(entity, value):
     # If the Hash already has a defined type_, exit early:
     if entity.type_:
         return
-    if not value:
+    if not value or not value.value:
         return
     # The `value` argument should be a HexBinary object, so we look at the
     # string in its `value` field.
diff --git a/setup.py b/setup.py
index 46f2aa3f..dbd82b49 100644
--- a/setup.py
+++ b/setup.py
@@ -25,8 +25,8 @@ def get_version():
     readme = f.read()
 
 install_requires = [
-    'importlib ; python_version=="2.6"',
-    'lxml>=2.2.3',
+    'lxml>=2.2.3 ; python_version == "2.7" or python_version >= "3.5"',
+    'lxml>=2.2.3, <4.4.0 ; python_version > "2.7" and python_version < "3.5"',
     'mixbox>=1.0.2',
     'python-dateutil',
 ]
@@ -47,10 +47,8 @@ def get_version():
         'License :: OSI Approved :: BSD License',
         'Operating System :: OS Independent',
         'Programming Language :: Python :: 2',
-        'Programming Language :: Python :: 2.6',
         'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.3',
         'Programming Language :: Python :: 3.4',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
diff --git a/tox.ini b/tox.ini
index d439b669..407bddc9 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = py26, py27, py33, py34, py35, py36, docs
+envlist = py27, py34, py35, py36, docs
 
 [testenv]
 commands =
@@ -13,7 +13,6 @@ commands =
 
 [travis]
 python =
-  2.6: py26
   2.7: py27, docs
   3.4: py34
   3.5: py35