diff --git a/httpretty/core.py b/httpretty/core.py
index 6d9870d..d6843e7 100644
--- a/httpretty/core.py
+++ b/httpretty/core.py
@@ -637,7 +637,7 @@ class Entry(BaseClass):
                 headers
             )
             headers.update({
-                'content-length': len(self.body)
+                'content-length': byte_type(len(self.body))
             })
 
         string_list = [