diff --git a/moto/s3/models.py b/moto/s3/models.py
index 4ce2afb3..525b3c81 100644
--- a/moto/s3/models.py
+++ b/moto/s3/models.py
@@ -9,6 +9,7 @@ import codecs
 import random
 import string
 import tempfile
+import sys
 
 import six
 
@@ -23,6 +24,7 @@ UPLOAD_ID_BYTES = 43
 UPLOAD_PART_MIN_SIZE = 5242880
 STORAGE_CLASS = ["STANDARD", "REDUCED_REDUNDANCY", "STANDARD_IA", "ONEZONE_IA"]
 DEFAULT_KEY_BUFFER_SIZE = 2 ** 24
+DEFAULT_TEXT_ENCODING = sys.getdefaultencoding()
 
 
 class FakeDeleteMarker(BaseModel):
@@ -74,6 +76,11 @@ class FakeKey(BaseModel):
     def value(self, new_value):
         self.value_buffer.seek(0)
         self.value_buffer.truncate()
+
+        # Hack for working around moto's own unit tests; this probably won't
+        # actually get hit in normal use.
+        if isinstance(new_value, six.text_type):
+            new_value = new_value.encode(DEFAULT_TEXT_ENCODING)
         self.value_buffer.write(new_value)
 
     def copy(self, new_name=None):
diff --git a/tox.ini b/tox.ini
index 0f3f1466..454409d7 100644
--- a/tox.ini
+++ b/tox.ini
@@ -2,6 +2,10 @@
 envlist = py27, py36
 
 [testenv]
+setenv =
+    BOTO_CONFIG=/dev/null
+    AWS_SECRET_ACCESS_KEY=foobar_secret
+    AWS_ACCESS_KEY_ID=foobar_key
 deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/requirements-dev.txt