diff --git a/pandas/io/tests/test_json/test_pandas.py b/pandas/io/tests/test_json/test_pandas.py
index 2889ace..8ccbb93 100644
--- a/pandas/io/tests/test_json/test_pandas.py
+++ b/pandas/io/tests/test_json/test_pandas.py
@@ -815,10 +815,49 @@ DataFrame\\.index values are different \\(100\\.0 %\\)
 
     def test_default_handler(self):
         value = object()
-        frame = DataFrame({'a': ['a', value]})
-        expected = frame.applymap(str)
-        result = pd.read_json(frame.to_json(default_handler=str))
+        frame = DataFrame({'a': [7, value]})
+        expected = DataFrame({'a': [7, value.id]})
+        result = pd.read_json(frame.to_json(default_handler=lambda x: x.id))
         assert_frame_equal(expected, result, check_index_type=False)
+        # Stringify by default.
+        expected = DataFrame({'a': [7, str(value)]})
+        result = pd.read_json(frame.to_json())
+        assert_frame_equal(expected, result, check_index_type=False)
+
+    def test_default_handler_with_json_dumps(self):
+        import json
+        import sys
+        print >>sys.stderr, "Testing 1 two"
+        class PlausibleHandler(json.JSONEncoder):
+            # disable method-hidden because
+            # https://github.com/PyCQA/pylint/issues/414
+            def default(self, obj): # pylint: disable=method-hidden
+                print >>sys.stderr, 'handling', obj
+                try:
+                    if hasattr(obj, 'to_json'):
+                        optargs = obj.to_json.__code__.co_varnames
+                        if 'default_handler' in optargs:
+                            print >>sys.stderr, 'Passing default_handler to to_json'
+                            return obj.to_json(default_handler=self.default)
+                        else:
+                            print >>sys.stderr, 'Just calling to_json'
+                            return obj.to_json()
+                    elif isinstance(obj, complex):
+                        return {'mathjs' : 'Complex',
+                                're' : x.real, 'im' : x.imag}
+                        # https://github.com/pydata/pandas/issues/12554
+                    return json.JSONEncoder.default(self, obj)
+                except Exception as e:
+                    return repr(e)
+        df_list = [ 9, DataFrame({'a': [1, 2.3, complex(4, -5)],
+                                  'b': [float('nan'), None, 'N/A']})]
+        expected = ('[9,{"a":{"0":1.0,"1":2.3,'
+                    '"2":{"mathjs":"Complex","re":4,"im":-5}},'
+                    '"b":{"0":null,"1":null,"2":"N\/A"}}]')
+        self.assertEqual(expected, json.dumps(df_list, cls=PlausibleHandler))
+        expected = ('{"a":{"0":1.0,"1":2.3,"2":"4-5j"},'
+                    '"b":{"0":null,"1":null,"2":"N\/A"}}')
+        self.assertEqual(expected, df_list[1].to_json())
 
     def test_default_handler_raises(self):
         def my_handler_raises(obj):
diff --git a/pandas/src/ujson/lib/ultrajson.h b/pandas/src/ujson/lib/ultrajson.h
index f83f74a..0149db8 100644
--- a/pandas/src/ujson/lib/ultrajson.h
+++ b/pandas/src/ujson/lib/ultrajson.h
@@ -155,6 +155,7 @@ enum JSTYPES
   JT_ARRAY,       // Array structure
   JT_OBJECT,    // Key/Value structure
   JT_INVALID,    // Internal, do not return nor expect
+  JT_UNHANDLED,  // Internal, do not return nor expect. Internally, try to use the defaultHandler if available.
 };
 
 typedef void * JSOBJ;
@@ -186,6 +187,12 @@ typedef struct __JSONObjectEncoder
   JSINT64 (*getLongValue)(JSOBJ obj, JSONTypeContext *tc);
   JSINT32 (*getIntValue)(JSOBJ obj, JSONTypeContext *tc);
   double (*getDoubleValue)(JSOBJ obj, JSONTypeContext *tc);
+  /*
+  Transform unhandled value in obj into something we can handle (like a string) in result.
+  Return a function pointer to free that object, which will be passed to it when finished,
+  or return NULL if transformation failed.
+  */
+  JSPFN_FREE (*transformUnhandledValue)(JSOBJ obj, JSONTypeContext *tc, JSOBJ* result);
 
   /*
   Begin iteration of an iteratable object (JS_ARRAY or JS_OBJECT)
diff --git a/pandas/src/ujson/lib/ultrajsonenc.c b/pandas/src/ujson/lib/ultrajsonenc.c
index 5e2a226..507e50b 100644
--- a/pandas/src/ujson/lib/ultrajsonenc.c
+++ b/pandas/src/ujson/lib/ultrajsonenc.c
@@ -870,6 +870,20 @@ void encode(JSOBJ obj, JSONObjectEncoder *enc, const char *name, size_t cbName)
       Buffer_AppendCharUnchecked (enc, '\"');
       break;
     }
+
+    default:
+    {
+      JSPFN_FREE liberator;
+      JSOBJ result;
+      liberator = enc->transformUnhandledValue(obj, &tc, &result);
+      if (liberator)
+      {
+        encode (result, enc, NULL, 0);
+        liberator(result);
+      }
+      break;
+    }
+
   }
 
   enc->endTypeContext(obj, &tc);
diff --git a/pandas/src/ujson/python/objToJSON.c b/pandas/src/ujson/python/objToJSON.c
index dcb509b..509f23b 100644
--- a/pandas/src/ujson/python/objToJSON.c
+++ b/pandas/src/ujson/python/objToJSON.c
@@ -158,8 +158,13 @@ enum PANDAS_FORMAT
   VALUES
 };
 
-//#define PRINTMARK() fprintf(stderr, "%s: MARK(%d)\n", __FILE__, __LINE__)
-#define PRINTMARK()
+// Debugging: uncomment the one you want.
+// for gcc:
+#define PRINTMARK() fprintf(stderr, "%s: MARK(%d)\t%s\n", __FILE__, __LINE__, __FUNCTION__)
+// for c99:
+//#define PRINTMARK() fprintf(stderr, "%s: MARK(%d)\t%s\n", __FILE__, __LINE__, __func__)
+// for normal operation:
+//#define PRINTMARK()
 
 int PdBlock_iterNext(JSOBJ, JSONTypeContext *);
 
@@ -471,6 +476,46 @@ static void *PyTimeToJSON(JSOBJ _obj, JSONTypeContext *tc, void *outValue, size_
   return outValue;
 }
 
+static void PyUnhandledFree(JSOBJ obj)
+{
+  PRINTMARK();
+  Py_DECREF((PyObject*)obj);
+}
+static JSPFN_FREE PyUnhandledTransformer(JSOBJ _obj, JSONTypeContext *tc, JSOBJ *_result)
+{
+  PyObject *obj = (PyObject *) _obj;
+  PyObjectEncoder *poenc = (PyObjectEncoder*) tc->encoder;
+  PyObject **result = (PyObject**) _result;
+  PyObject_Print(obj, stderr, 0);
+  PyObject_Print(poenc->enc.iterGetValue(_obj, tc), stderr, 0);
+  if (poenc->defaultHandler)
+  {
+    PRINTMARK();
+    *result = PyObject_CallFunctionObjArgs(poenc->defaultHandler, obj, NULL);
+  }
+  else
+  {
+    PRINTMARK();
+    *result = PyObject_Str(obj);
+  }
+  if (*result == NULL || PyErr_Occurred())
+  {
+    PRINTMARK();
+    if (!PyErr_Occurred())
+    {
+	PyErr_SetString(PyExc_TypeError, "Failed to execute default handler");
+    }
+    return NULL;
+  }
+  if (*result == obj)
+  {
+    PyErr_SetString(PyExc_TypeError, "Default handler returned same object as it received");
+    return NULL;
+  }
+  PRINTMARK();
+  return &PyUnhandledFree;
+}
+
 static int NpyTypeToJSONType(PyObject* obj, JSONTypeContext* tc, int npyType, void* value)
 {
   PyArray_VectorUnaryFunc* castfunc;
@@ -544,12 +589,10 @@ static int NpyTypeToJSONType(PyObject* obj, JSONTypeContext* tc, int npyType, vo
     return *((npy_bool *) value) == NPY_TRUE ? JT_TRUE : JT_FALSE;
   }
 
-  PRINTMARK();
-  PyErr_Format (
-      PyExc_RuntimeError,
-      "Unhandled numpy dtype %d",
-      npyType);
-  return JT_INVALID;
+  PyObject_Print(value, stderr, 0);
+  PRINTMARK();  // GREMIO
+  PyObject_Print(obj, stderr, 0);
+  return JT_UNHANDLED;
 }
 
 
@@ -2395,6 +2438,7 @@ ISITERABLE:
 
   PyErr_Clear();
 
+  /*
   if (enc->defaultHandler)
   {
     PRINTMARK();
@@ -2411,6 +2455,7 @@ ISITERABLE:
     Py_DECREF(tmpObj);
     goto INVALID;
   }
+  */
 
   PRINTMARK();
   tc->type = JT_OBJECT;
@@ -2531,6 +2576,7 @@ PyObject* objToJSON(PyObject* self, PyObject *args, PyObject *kwargs)
         Object_getLongValue,
         Object_getIntValue,
         Object_getDoubleValue,
+        PyUnhandledTransformer,
         Object_iterBegin,
         Object_iterNext,
         Object_iterEnd,