diff --git a/Modules/zlibmodule.c b/Modules/zlibmodule.c
--- a/Modules/zlibmodule.c
+++ b/Modules/zlibmodule.c
@@ -55,6 +55,9 @@ zlib_error(z_stream zst, int err, char *
     const char *zmsg = zst.msg;
     if (zmsg == Z_NULL) {
         switch (err) {
+        /* Z_OK in unflush() means we never get to Z_STREAM_END,
+           so it must be that the stream is truncated */
+        case Z_OK:
         case Z_BUF_ERROR:
             zmsg = "incomplete or truncated stream";
             break;
@@ -697,7 +700,7 @@ PyZlib_flush(compobject *self, PyObject 
     if (err == Z_STREAM_END && flushmode == Z_FINISH) {
         err = deflateEnd(&(self->zst));
         if (err != Z_OK) {
-            zlib_error(self->zst, err, "from deflateEnd()");
+            zlib_error(self->zst, err, "while finishing compression");
             Py_DECREF(RetVal);
             RetVal = NULL;
             goto error;
@@ -881,18 +884,22 @@ PyZlib_unflush(compobject *self, PyObjec
         Py_END_ALLOW_THREADS
     }
 
-    /* If flushmode is Z_FINISH, we also have to call deflateEnd() to free
-       various data structures. Note we should only get Z_STREAM_END when
-       flushmode is Z_FINISH */
+    /* As flushmode is Z_FINISH, we also have to call inflateEnd() to free
+       various data structures */
     if (err == Z_STREAM_END) {
         err = inflateEnd(&(self->zst));
         self->is_initialised = 0;
         if (err != Z_OK) {
-            zlib_error(self->zst, err, "from inflateEnd()");
+            zlib_error(self->zst, err, "while finishing data decompression");
             Py_DECREF(retval);
             retval = NULL;
             goto error;
         }
+    } else {
+        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);