diff --git a/zipline/_protocol.pyx b/zipline/_protocol.pyx
index d2b1a220..2eb48f23 100644
--- a/zipline/_protocol.pyx
+++ b/zipline/_protocol.pyx
@@ -82,48 +82,40 @@ cdef class check_parameters(object):
             for i, arg in enumerate(args[1:]):
                 expected_type = self.types[i]
 
-                if isinstance(arg, expected_type):
-                    continue
-
-                elif (i == 0 or i == 1) and _is_iterable(arg):
+                if (i == 0 or i == 1) and _is_iterable(arg):
                     if len(arg) == 0:
                         continue
+                    arg = arg[0]
 
-                    if isinstance(arg[0], expected_type):
-                        continue
+                if not isinstance(arg, expected_type):
+                    expected_type_name = expected_type.__name__ \
+                        if not _is_iterable(expected_type) \
+                        else ', '.join([type_.__name__ for type_ in expected_type])
 
-                expected_type_name = expected_type.__name__ \
-                    if not _is_iterable(expected_type) \
-                    else ', '.join([type_.__name__ for type_ in expected_type])
-
-                raise TypeError("Expected %s argument to be of type %s%s" %
-                                (self.keyword_names[i],
-                                 'or iterable of type ' if i in (0, 1) else '',
-                                 expected_type_name)
-                )
+                    raise TypeError("Expected %s argument to be of type %s%s" %
+                        (self.keyword_names[i],
+                         'or iterable of type ' if i in (0, 1) else '',
+                         expected_type_name)
+                    )
 
             # verify type of each kwarg
             for keyword, arg in iteritems(kwargs):
-                if isinstance(arg, self.keys_to_types[keyword]):
-                    continue
-                elif keyword in ('assets', 'fields') and _is_iterable(arg):
+                if keyword in ('assets', 'fields') and _is_iterable(arg):
                     if len(arg) == 0:
                         continue
+                    arg = arg[0]
+                if not isinstance(arg, self.keys_to_types[keyword]):
+                    expected_type = self.keys_to_types[keyword].__name__ \
+                        if not _is_iterable(self.keys_to_types[keyword]) \
+                        else ', '.join([type_.__name__ for type_ in
+                            self.keys_to_types[keyword]])
 
-                    if isinstance(arg[0], self.keys_to_types[keyword]):
-                        continue
-
-                expected_type = self.keys_to_types[keyword].__name__ \
-                    if not _is_iterable(self.keys_to_types[keyword]) \
-                    else ', '.join([type_.__name__ for type_ in
-                                    self.keys_to_types[keyword]])
-
-                raise TypeError("Expected %s argument to be of type %s%s" %
-                                (keyword,
-                                 'or iterable of type ' if keyword in
-                                 ('assets', 'fields') else '',
-                                 expected_type)
-                )
+                    raise TypeError("Expected %s argument to be of type %s%s" %
+                                    (keyword,
+                                     'or iterable of type ' if keyword in
+                                     ('assets', 'fields') else '',
+                                     expected_type)
+                    )
 
             return func(*args, **kwargs)