BUG: passing an invalid fill method to resample(..).fillna() causes an odd error message

doc/source/whatsnew/v0.18.1.txt

@@ -280,7 +280,7 @@ Bug Fixes
 
 - Bug in ``.drop()`` with a non-unique ``MultiIndex``. (:issue:`12701`)
 - Bug in ``.concat`` of datetime tz-aware and naive DataFrames (:issue:`12467`)
-
+- Bug in correctly raising a ``ValueError`` in ``.resample(..).fillna(..)`` when passing a non-string (:issue:`12952`)
 
 - Bug in ``Timestamp.__repr__`` that caused ``pprint`` to fail in nested structures (:issue:`12622`)
 - Bug in ``Timedelta.min`` and ``Timedelta.max``, the properties now report the true minimum/maximum ``timedeltas`` as recognized by Pandas. See :ref:`documentation <timedeltas.limitations>`. (:issue:`12727`)

pandas/core/missing.py

@@ -7,7 +7,7 @@
 import pandas.core.common as com
 import pandas.algos as algos
 import pandas.lib as lib
-from pandas.compat import range
+from pandas.compat import range, string_types
 
 
 def mask_missing(arr, values_to_mask):
@@ -60,11 +60,13 @@ def mask_missing(arr, values_to_mask):
 def clean_fill_method(method, allow_nearest=False):
     if method is None:
         return None
-    method = method.lower()
-    if method == 'ffill':
-        method = 'pad'
-    if method == 'bfill':
-        method = 'backfill'
+
+    if isinstance(method, string_types):
+        method = method.lower()
+        if method == 'ffill':
+            method = 'pad'
+        elif method == 'bfill':
+            method = 'backfill'
 
     valid_methods = ['pad', 'backfill']
     expecting = 'pad (ffill) or backfill (bfill)'

pandas/tseries/tests/test_resample.py

@@ -356,6 +356,9 @@ def test_fillna(self):
         result = r.fillna(method='bfill')
         assert_series_equal(result, expected)
 
+        with self.assertRaises(ValueError):
+            r.fillna(0)
+
     def test_apply_without_aggregation(self):
 
         # both resample and groupby should work w/o aggregation