diff --git a/sympy/core/numbers.py b/sympy/core/numbers.py
index a21bfec58bda..78ef86a56a06 100644
--- a/sympy/core/numbers.py
+++ b/sympy/core/numbers.py
@@ -56,10 +56,16 @@ def seterr(divide=False):
 
 def _decimal_to_Rational_prec(dec):
     """Convert an ordinary decimal instance to a Rational."""
-    assert not dec._is_special
+    # _is_special is needed for Python 2.5 support; is_finite for Python 3.3
+    # support
+    nonfinite = getattr(dec, '_is_special', None)
+    if nonfinite is None:
+        nonfinite = not dec.is_finite()
+    if nonfinite:
+        raise TypeError("dec must be finite, got %s." % dec)
     s, d, e = dec.as_tuple()
     prec = len(d)
-    if dec._isinteger():
+    if int(dec) == dec:
         rv = Rational(int(dec))
     else:
         s = (-1)**s