diff --git a/src/xpra/client/gl/gl_window_backing.py b/src/xpra/client/gl/gl_window_backing.py
index 64aef8430a..d068003a7d 100644
--- a/src/xpra/client/gl/gl_window_backing.py
+++ b/src/xpra/client/gl/gl_window_backing.py
@@ -366,20 +366,20 @@ def _do_paint_rgb(self, bpp, img_data, x, y, width, height, rowstride, options,
 
             self.gl_marker("RGB%s update at %d,%d, size %d,%d, stride is %d, row length %d, alignment %d" % (bpp, x, y, width, height, rowstride, row_length, alignment))
             # Upload data as temporary RGB texture
-            rgb_format = options.get("rgb_format", "RGB")
+            rgb_format = options.get("rgb_format", None)
             log.info("options=%s, rgb format=%s", options, rgb_format)
             if bpp==24:
                 if rgb_format=="BGR":
                     pformat = GL_BGR
                 else:
-                    assert rgb_format=="RGB"
+                    assert rgb_format in ("RGB", None), "invalid 24-bit format: %s" % rgb_format
                     pformat = GL_RGB
             else:
                 assert bpp==32
                 if rgb_format=="BGRA":
                     pformat = GL_BGRA
                 else:
-                    assert rgb_format=="RGBA"
+                    assert rgb_format in ("RGBA", None), "invalid 32-bit format: %s" % rgb_format
                     pformat = GL_RGBA
 
             glBindTexture(GL_TEXTURE_RECTANGLE_ARB, self.textures[TEX_RGB])