diff --git a/prefixed/__init__.py b/prefixed/__init__.py
index d189171..3dee5ba 100644
--- a/prefixed/__init__.py
+++ b/prefixed/__init__.py
@@ -15,7 +15,7 @@
 import re
 import sys
 
-__version__ = '0.4.0'
+__version__ = '0.4.1'
 
 try:
     BASESTRING = basestring
@@ -104,6 +104,7 @@ def raise_from_none(exc):  # pragma: no cover
 else:
     from string import maketrans  # pragma: no cover
 
+DEPRECATED = {'j', 'J'}
 TRANS_DEPRECATED = maketrans('jJ', 'km')
 
 
@@ -299,13 +300,14 @@ def __format__(self, format_spec):
 
         spec = match.groupdict()
 
+        # Handle deprecated spec types
+        if spec['type'] in DEPRECATED:
+            spec['type'] = spec['type'].translate(TRANS_DEPRECATED)
+
         # If not a spec we handle, use float.__format__(()
         if spec['type'] not in {'h', 'H', 'k', 'K', 'm', 'M'}:
             return super(Float, self).__format__(format_spec)
 
-        # Handle deprecated spec types
-        spec['type'] = spec['type'].translate(TRANS_DEPRECATED)
-
         # Determine value and prefix
         value, prefix, spec = _convert(float(self), spec)
 
diff --git a/tests/test_float.py b/tests/test_float.py
index c7552c7..915505f 100644
--- a/tests/test_float.py
+++ b/tests/test_float.py
@@ -418,6 +418,14 @@ def test_margin(self):
         self.assertEqual(format(Float(1049), '%5.2h'), '1049.00')
         self.assertEqual(format(Float(1050), '%5.2h'), '1.05k')
 
+    def test_deprecated(self):
+        """
+        Confirm deprecated format specifiers function
+        """
+
+        self.assertEqual(format(Float(2048), '.2j'), '2.00Ki')
+        self.assertEqual(format(Float(2048), '.2J'), '2.00K')
+
 
 class TestFloatMath(unittest.TestCase):
     """