How to fix flash with image effect when enabled?

Hi, I'm using the following code to combine two different images from different cameras. It works fine except for 1 frame when the image effect is enabled - there is a flash where the skytrainCam renders straight to the screen without blending. Can anyone suggest why this flash may be happening?

Thanks.

function OnRenderImage (source : RenderTexture, destination : RenderTexture) {

         if (!camera.enabled || !isActive){
             Graphics.Blit(source, destination);
             return;
         }
         
         var skytrainRender : RenderTexture = RenderTexture.GetTemporary(source.width, source.height, 16);
         var finalRender : RenderTexture = RenderTexture.GetTemporary(source.width, source.height, 16);
         
         skytrainCam.enabled = false;
         skytrainCam.targetTexture = skytrainRender;
         skytrainCam.Render();
         skytrainCam.targetTexture = null;
         
         // apply skytrain layer as regular render
         Graphics.Blit(skytrainRender, finalRender);
         
         // apply regular camera layer at half alpha
         Graphics.Blit(source, finalRender, combineMaterial);
         
         // blit to destination to finish it off...
         Graphics.Blit(finalRender, destination);
         
         // release textures
         RenderTexture.ReleaseTemporary (finalRender);
         RenderTexture.ReleaseTemporary (skytrainRender);
     }