diff --git a/moto/s3/responses.py b/moto/s3/responses.py
index 80a0a9421..5f1be0fbb 100644
--- a/moto/s3/responses.py
+++ b/moto/s3/responses.py
@@ -80,19 +80,17 @@ def key_response(uri_info, method, body, headers):
             s3_backend.copy_key(src_bucket, src_key, bucket_name, key_name)
             template = Template(S3_OBJECT_COPY_RESPONSE)
             return template.render(key=src_key)
-        if body is not None:
-            key = s3_backend.get_key(bucket_name, key_name)
-            if not key or body:
-                # We want to write the key in once of two circumstances.
-                # - The key does not currently exist.
-                # - The key already exists, but body is a truthy value.
-                # This allows us to write empty strings to keys for the first
-                # write, but not subsequent. This is because HTTPretty sends
-                # an empty string on connection close. This is a temporary fix
-                # while HTTPretty gets fixed.
-                new_key = s3_backend.set_key(bucket_name, key_name, body)
-                template = Template(S3_OBJECT_RESPONSE)
-                return template.render(key=new_key), dict(etag=new_key.etag)
+        content_length = int(headers.get('Content-Length', 0))
+        if body or (body == '' and content_length == 0):
+            # We want to write the key in once of two circumstances.
+            # - Anytime we are given a truthy body value
+            # - We are given an empty body value and the content length is zero.
+            # The reason we do not set the key to an empty string if the
+            # content length is not zero is because we are sometimes sent an
+            # empty string as part of closing the connection.
+            new_key = s3_backend.set_key(bucket_name, key_name, body)
+            template = Template(S3_OBJECT_RESPONSE)
+            return template.render(key=new_key), dict(etag=new_key.etag)
         key = s3_backend.get_key(bucket_name, key_name)
         if key:
             return "", dict(etag=key.etag)
diff --git a/tests/test_s3/test_s3.py b/tests/test_s3/test_s3.py
index 31e011bfc..278ce9e2f 100644
--- a/tests/test_s3/test_s3.py
+++ b/tests/test_s3/test_s3.py
@@ -62,6 +62,20 @@ def test_empty_key():
     bucket.get_key("the-key").get_contents_as_string().should.equal('')
 
 
+@mock_s3
+def test_empty_key_set_on_existing_key():
+    conn = boto.connect_s3('the_key', 'the_secret')
+    bucket = conn.create_bucket("foobar")
+    key = Key(bucket)
+    key.key = "the-key"
+    key.set_contents_from_string("foobar")
+
+    bucket.get_key("the-key").get_contents_as_string().should.equal('foobar')
+
+    key.set_contents_from_string("")
+    bucket.get_key("the-key").get_contents_as_string().should.equal('')
+
+
 @mock_s3
 def test_copy_key():
     conn = boto.connect_s3('the_key', 'the_secret')