1 files changed, 13 insertions, 3 deletions
diff --git a/Modules/_json.c b/Modules/_json.c
index 59376a7b0f..8f1743bada 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -1431,10 +1431,20 @@ static PyObject *
 encoder_encode_string(PyEncoderObject *s, PyObject *obj)
 {
     /* Return the JSON representation of a string */
-    if (s->fast_encode)
+    PyObject *encoded;
+
+    if (s->fast_encode) {
         return s->fast_encode(NULL, obj);
-    else
-        return PyObject_CallFunctionObjArgs(s->encoder, obj, NULL);
+    }
+    encoded = PyObject_CallFunctionObjArgs(s->encoder, obj, NULL);
+    if (encoded != NULL && !PyUnicode_Check(encoded)) {
+        PyErr_Format(PyExc_TypeError,
+                     "encoder() must return a string, not %.80s",
+                     Py_TYPE(encoded)->tp_name);
+        Py_DECREF(encoded);
+        return NULL;
+    }
+    return encoded;
 }
 
 static int