diff --git a/Lib/test/test_zlib.py b/Lib/test/test_zlib.py
--- a/Lib/test/test_zlib.py
+++ b/Lib/test/test_zlib.py
@@ -446,6 +446,13 @@ class CompressObjectTestCase(BaseCompres
         y = dco.decompress(x[:-5])
         y += dco.flush()
         self.assertEqual(y, b'foo')
+        # flush(strict=True) raises an error, though
+        dco = zlib.decompressobj()
+        dco.decompress(x[:-5])
+        with self.assertRaises(zlib.error) as cm:
+            dco.flush(strict=True)
+        self.assertEqual(str(cm.exception),
+            "Error -5 while decompressing data: incomplete or truncated stream")
 
     if hasattr(zlib.compressobj(), "copy"):
         def test_compresscopy(self):
diff --git a/Modules/zlibmodule.c b/Modules/zlibmodule.c
--- a/Modules/zlibmodule.c
+++ b/Modules/zlibmodule.c
@@ -838,13 +838,16 @@ PyDoc_STRVAR(decomp_flush__doc__,
 "The decompressor object can no longer be used after this call.");
 
 static PyObject *
-PyZlib_unflush(compobject *self, PyObject *args)
+PyZlib_unflush(compobject *self, PyObject *args, PyObject *kwargs)
 {
+    static char *kwlist[] = {"length", "strict", NULL};
     int err, length = DEFAULTALLOC;
     PyObject * retval = NULL;
     unsigned long start_total_out;
+    int strict = 0;
 
-    if (!PyArg_ParseTuple(args, "|i:flush", &length))
+    if (!PyArg_ParseTupleAndKeywords(args, kwargs, "|ii:flush",
+        kwlist, &length, &strict))
         return NULL;
     if (length <= 0) {
         PyErr_SetString(PyExc_ValueError, "length must be greater than zero");
@@ -866,7 +869,7 @@ PyZlib_unflush(compobject *self, PyObjec
 
     /* while Z_OK and the output buffer is full, there might be more output,
        so extend the output buffer and try again */
-    while ((err == Z_OK || err == Z_BUF_ERROR) && self->zst.avail_out == 0) {
+    while (err == Z_OK || (err == Z_BUF_ERROR && self->zst.avail_out == 0)) {
         if (_PyBytes_Resize(&retval, length << 1) < 0) {
             Py_DECREF(retval);
             retval = NULL;
@@ -894,6 +897,12 @@ PyZlib_unflush(compobject *self, PyObjec
             goto error;
         }
     }
+    else if (strict) {
+        zlib_error(self->zst, err, "while decompressing data");
+        Py_DECREF(retval);
+        retval = NULL;
+        goto error;
+    }
     if (_PyBytes_Resize(&retval, self->zst.total_out - start_total_out) < 0) {
         Py_DECREF(retval);
         retval = NULL;
@@ -923,7 +932,7 @@ static PyMethodDef Decomp_methods[] =
 {
     {"decompress", (binaryfunc)PyZlib_objdecompress, METH_VARARGS,
                    decomp_decompress__doc__},
-    {"flush", (binaryfunc)PyZlib_unflush, METH_VARARGS,
+    {"flush", (binaryfunc)PyZlib_unflush, METH_VARARGS | METH_KEYWORDS,
               decomp_flush__doc__},
 #ifdef HAVE_ZLIB_COPY
     {"copy",  (PyCFunction)PyZlib_uncopy, METH_NOARGS,