diff --git a/Lib/json/decoder.py b/Lib/json/decoder.py
index f0bc245..4e88ba6 100644
--- a/Lib/json/decoder.py
+++ b/Lib/json/decoder.py
@@ -18,11 +18,15 @@
 
 
 def linecol(doc, pos):
-    lineno = doc.count('\n', 0, pos) + 1
+    if isinstance(doc, bytes):
+        newline = b'\n'
+    else:
+        newline = '\n'
+    lineno = doc.count(newline, 0, pos) + 1
     if lineno == 1:
         colno = pos
     else:
-        colno = pos - doc.rindex('\n', 0, pos)
+        colno = pos - doc.rindex(newline, 0, pos)
     return lineno, colno
 
 
diff --git a/Lib/json/tests/test_scanstring.py b/Lib/json/tests/test_scanstring.py
index cd205a4..025d15d 100644
--- a/Lib/json/tests/test_scanstring.py
+++ b/Lib/json/tests/test_scanstring.py
@@ -2,6 +2,7 @@
 import decimal
 from unittest import TestCase
 
+import json
 import json.decoder
 
 class TestScanString(TestCase):
@@ -101,3 +102,9 @@
         self.assertEquals(
             scanstring('["Bad value", truth]', 2, None, True),
             ('Bad value', 12))
+
+    def test_issue3623(self):
+        self.assertRaises(ValueError, json.decoder.scanstring, b"xxx", 1,
+                          "xxx")
+        self.assertRaises(UnicodeDecodeError,
+                          json.encoder.encode_basestring_ascii, b"xx\xff")
diff --git a/Modules/_json.c b/Modules/_json.c
index 47c4a56..0068bda 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -179,11 +179,13 @@
         errmsg_fn = PyObject_GetAttrString(decoder, "errmsg");
         if (errmsg_fn == NULL)
             return;
-        Py_XDECREF(decoder);
+        Py_DECREF(decoder);
     }
     pymsg = PyObject_CallFunction(errmsg_fn, "(zOn)", msg, s, end);
-    PyErr_SetObject(PyExc_ValueError, pymsg);
-    Py_DECREF(pymsg);
+    if (pymsg) {
+        PyErr_SetObject(PyExc_ValueError, pymsg);
+        Py_DECREF(pymsg);
+    }
 /*
 
 def linecol(doc, pos):
@@ -602,7 +604,7 @@
                      Py_TYPE(pystr)->tp_name);
         return NULL;
     }
-    if (PyBytes_Check(rval)) {
+    if (rval != NULL && PyBytes_Check(rval)) {
         PyObject *urval = PyUnicode_DecodeASCII(PyBytes_AS_STRING(rval), PyBytes_GET_SIZE(rval), NULL);
         Py_DECREF(rval);
         return urval;