diff --git a/Modules/_io/_iomodule.c b/Modules/_io/_iomodule.c
index 733a7b95d72..a1c451eb566 100644
--- a/Modules/_io/_iomodule.c
+++ b/Modules/_io/_iomodule.c
@@ -451,7 +451,7 @@ io_open(PyObject *self, PyObject *args, PyObject *kwds)
             if (fileno == -1 && PyErr_Occurred())
                 goto error;
 
-            if (fstat(fileno, &st) >= 0)
+            if (fstat(fileno, &st) >= 0 && st.st_blksize > 1)
                 buffering = st.st_blksize;
         }
 #endif