diff --git a/lib/spring/application.rb b/lib/spring/application.rb
index e19b8b5b..d41917df 100644
--- a/lib/spring/application.rb
+++ b/lib/spring/application.rb
@@ -134,16 +134,19 @@ def run
     end
 
     def with_env_vars(env)
-      # Delete all env vars which are unchanged from before spring started
-      original_env.each { |k, v| ENV.delete k if ENV[k] == v }
+      # Allowed are keys currently not in ENV...
+      allowed_keys = env.keys - ENV.keys
+      # ...and ENV-keys whose values have not changed since the start of spring
+      allowed_keys += original_env.select {|k, v| ENV[k] == v }.keys
+      # never allowed:
+      allowed_keys -= %w(RUBYOPT RUBY_ROOT BUNDLE_GEMFILE GEM_ROOT GEM_HOME GEM_PATH)
+      allowed_keys.uniq!
 
-      changed_keys = env.keys - ENV.keys
-      # Load in the current env vars, except those which *were* changed when spring started
-      env.each { |k, v| ENV[k] ||= v }
+      allowed_keys.each{|k| ENV[k] = env[k] }
 
       yield
     ensure
-      changed_keys.each do |k|
+      allowed_keys.each do |k|
         original_env.has_key?(k) ? ENV[k] = original_env[k] : ENV.delete(k)
       end
     end