diff --git a/doc/source/release.rst b/doc/source/release.rst
index a2015a3b361ac..f1f685850284c 100644
--- a/doc/source/release.rst
+++ b/doc/source/release.rst
@@ -633,6 +633,7 @@ Bug Fixes
   - Fixed issue with ``drop`` and a non-unique index on Series (:issue:`5248`)
   - Fixed seg fault in C parser caused by passing more names than columns in
     the file. (:issue:`5156`)
+  - More informative exception when trying to use ``MS`` as period frequency (:issue:`5332`)
 
 pandas 0.12.0
 -------------
diff --git a/pandas/tseries/frequencies.py b/pandas/tseries/frequencies.py
index cfe874484231b..a7e315af5c1cc 100644
--- a/pandas/tseries/frequencies.py
+++ b/pandas/tseries/frequencies.py
@@ -337,7 +337,7 @@ def get_base_alias(freqstr):
     """
     return _base_and_stride(freqstr)[0]
 
-_dont_uppercase = ['MS', 'ms']
+_dont_uppercase = set(('MS', 'ms'))
 
 
 def get_offset(name):
@@ -497,7 +497,7 @@ def _period_alias_dictionary():
     H_aliases = ["H", "HR", "HOUR", "HRLY", "HOURLY"]
     T_aliases = ["T", "MIN", "MINUTE", "MINUTELY"]
     S_aliases = ["S", "SEC", "SECOND", "SECONDLY"]
-    L_aliases = ["L", "MS", "MILLISECOND", "MILLISECONDLY"]
+    L_aliases = ["L", "ms", "MILLISECOND", "MILLISECONDLY"]
     U_aliases = ["U", "US", "MICROSECOND", "MICROSECONDLY"]
     N_aliases = ["N", "NS", "NANOSECOND", "NANOSECONDLY"]
 
@@ -615,10 +615,13 @@ def _period_group(freqstr):
 def _period_str_to_code(freqstr):
     # hack
     freqstr = _rule_aliases.get(freqstr, freqstr)
-    freqstr = _rule_aliases.get(freqstr.lower(), freqstr)
+    
+    if freqstr not in _dont_uppercase:
+        freqstr = _rule_aliases.get(freqstr.lower(), freqstr)
 
     try:
-        freqstr = freqstr.upper()
+        if freqstr not in _dont_uppercase:
+            freqstr = freqstr.upper()
         return _period_code_map[freqstr]
     except KeyError:
         try:
diff --git a/pandas/tseries/tests/test_period.py b/pandas/tseries/tests/test_period.py
index 312a88bcbc5a9..84b62e7931156 100644
--- a/pandas/tseries/tests/test_period.py
+++ b/pandas/tseries/tests/test_period.py
@@ -13,7 +13,7 @@
 from numpy.ma.testutils import assert_equal
 
 from pandas import Timestamp
-from pandas.tseries.frequencies import MONTHS, DAYS
+from pandas.tseries.frequencies import MONTHS, DAYS, _period_code_map
 from pandas.tseries.period import Period, PeriodIndex, period_range
 from pandas.tseries.index import DatetimeIndex, date_range, Index
 from pandas.tseries.tools import to_datetime
@@ -488,7 +488,14 @@ def test_constructor_infer_freq(self):
 
         p = Period('2007-01-01 07:10:15.123400')
         self.assert_(p.freq == 'U')
-
+        
+    def test_asfreq_MS(self):
+        initial = Period("2013")
+        
+        self.assertEqual(initial.asfreq(freq="M", how="S"), Period('2013-01', 'M'))
+        self.assertRaises(ValueError, initial.asfreq, freq="MS", how="S")
+        tm.assertRaisesRegexp(ValueError, "Unknown freqstr: MS", pd.Period, '2013-01', 'MS')
+        self.assertTrue(_period_code_map.get("MS") is None)
 
 def noWrap(item):
     return item