diff --git a/pipenv/project.py b/pipenv/project.py
index 7f40576d99..d0d21c3eca 100644
--- a/pipenv/project.py
+++ b/pipenv/project.py
@@ -14,6 +14,7 @@
 import six
 import toml
 import json as simplejson
+from prettytoml.elements.array import ArrayElement
 
 from ._compat import Path
 
@@ -64,6 +65,12 @@ def _normalized(p):
 DEFAULT_NEWLINES = u"\n"
 
 
+def encode_toml_elements(obj):
+    if isinstance(obj, ArrayElement):
+        return obj.primitive_value
+    raise TypeError(repr(obj) + " is not JSON serializable")
+
+
 def preferred_newlines(f):
     if isinstance(f.newlines, six.text_type):
         return f.newlines
@@ -631,7 +638,8 @@ def write_lockfile(self, content):
         """
         newlines = self._lockfile_newlines
         s = simplejson.dumps(  # Send Unicode in to guarentee Unicode out.
-            content, indent=4, separators=(u",", u": "), sort_keys=True
+            content, indent=4, separators=(u",", u": "), sort_keys=True,
+            default=encode_toml_elements,
         )
         with atomic_open_for_write(self.lockfile_location, newline=newlines) as f:
             f.write(s)