diff -r a6af8ea3488b jython/Lib/test/test_float_jy.py
--- a/jython/Lib/test/test_float_jy.py	Sun May 18 23:36:01 2008 -0400
+++ b/jython/Lib/test/test_float_jy.py	Sat May 24 00:42:01 2008 -0400
@@ -59,6 +59,9 @@ class FloatTestCase(unittest.TestCase):
         self.assertEqual('%.2g' % 99, '99')
         self.assertEqual('%.2g' % 100, '1e+02')
 
+    def test_float_none(self):
+        self.assertRaises(TypeError, float, None)
+
 
 def test_main():
     test_support.run_unittest(FloatTestCase)
diff -r a6af8ea3488b jython/src/org/python/core/PyFloat.java
--- a/jython/src/org/python/core/PyFloat.java	Sun May 18 23:36:01 2008 -0400
+++ b/jython/src/org/python/core/PyFloat.java	Sat May 24 00:42:01 2008 -0400
@@ -23,16 +23,33 @@ public class PyFloat extends PyObject
             PyObject[] args, String[] keywords) {
         ArgParser ap = new ArgParser("float", args, keywords, new String[] { "x" }, 0);
         PyObject x = ap.getPyObject(0, null);
-        if (new_.for_type == subtype) {
-            if (x == null) {
+        if (x == null) {
+            if (new_.for_type == subtype) {
                 return new PyFloat(0.0);
-            }
-            return x.__float__();
-        } else {
-            if (x == null) {
+            } else {
                 return new PyFloatDerived(subtype, 0.0);
             }
-            return new PyFloatDerived(subtype, x.__float__().getValue());
+        } else {
+            PyFloat floatObject = null;
+            try {
+                floatObject = x.__float__();
+            } catch (PyException e) {
+                if (Py.matchException(e, Py.AttributeError)) {
+                    // Translate AttributeError to TypeError
+                    // XXX: We are using the same message as CPython, even if
+                    //      it is not strictly correct (instances of types
+                    //      that implement the __float__ method are also
+                    //      valid arguments)
+                    throw Py.TypeError(
+                        "float() argument must be a string or a number");
+                }
+                throw e;
+            }
+            if (new_.for_type == subtype) {
+                return floatObject;
+            } else {
+                return new PyFloatDerived(subtype, floatObject.getValue());
+            }
         }
     }