Fix parser

d3/model/formats/obj.py

@@ -55,12 +55,15 @@ class OBJParser(TextModelParser):
 
             for i in range(len(splits)):
                 for j in range(len(splits[i])):
-                    if splits[i][j] is not '':
+                    if splits[i][j] != '':
                         splits[i][j] = int(splits[i][j])
                         if splits[i][j] > 0:
                             splits[i][j] -= 1
                         else:
                             splits[i][j] = len(self.vertices) + splits[i][j]
+                    elif j == 1:
+                        # Here, splits[i][j] == '', so we must set the value to None
+                        splits[i][j] = None
 
             # if Face3
             if len(split) == 3: