diff --git a/src/mesa/state_tracker/st_cb_fbo.c b/src/mesa/state_tracker/st_cb_fbo.c
index 20894825f7f..62f23a15ec9 100644
--- a/src/mesa/state_tracker/st_cb_fbo.c
+++ b/src/mesa/state_tracker/st_cb_fbo.c
@@ -648,7 +648,8 @@ st_ReadBuffer(struct gl_context *ctx, GLenum buffer)
    (void) buffer;
 
    /* add the renderbuffer on demand */
-   st_manager_add_color_renderbuffer(st, fb, fb->_ColorReadBufferIndex);
+   if (fb->_ColorReadBufferIndex >= 0)
+      st_manager_add_color_renderbuffer(st, fb, fb->_ColorReadBufferIndex);
 }