1 files changed, 1 insertions, 1 deletions
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index 4cde5f87032..61cfec435fe 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -607,9 +607,9 @@ _io_BytesIO_readinto_impl(bytesio *self, Py_buffer *buffer)
             len = 0;
     }
 
-    memcpy(buffer->buf, PyBytes_AS_STRING(self->buf) + self->pos, len);
     assert(self->pos + len < PY_SSIZE_T_MAX);
     assert(len >= 0);
+    memcpy(buffer->buf, PyBytes_AS_STRING(self->buf) + self->pos, len);
     self->pos += len;
 
     return PyLong_FromSsize_t(len);