I agree with others that this is not what the rules say. Implementing such a rule would be a houserule.

However, I believe it would be a good houserule. I don't think that Spell Resistance is supposed to be a pain in the behind to the creature with it. And an improvement to spell resistance amounts to an indirect nerf to spellcasters, so there shouldn't be any major balance issues.

The rule has a slight indirect improvement to balance, and removes an unneeded headache from the game. Houserule away.