diff --git a/lib/mime/type/columnar.rb b/lib/mime/type/columnar.rb
index 7006fab..2d3fa43 100644
--- a/lib/mime/type/columnar.rb
+++ b/lib/mime/type/columnar.rb
@@ -54,7 +54,7 @@ def encode_with(coder) # :nodoc:
   end
 
   def update_sort_priority
-    if @container.__fully_loaded?
+    if @container.__fully_loaded? || @__sort_priority.nil?
       super
     else
       obsolete = (@__sort_priority & (1 << 7)) != 0
diff --git a/test/test_mime_types_class.rb b/test/test_mime_types_class.rb
index 05494dd..4904c4c 100644
--- a/test/test_mime_types_class.rb
+++ b/test/test_mime_types_class.rb
@@ -103,6 +103,13 @@ def setup
       assert_includes MIME::Types.type_for("xtxt"), "text/plain"
     end
 
+    it "sorts correctly after modifying type extensions" do
+      a = MIME::Types["text/plain"].first
+      b = MIME::Types["image/jpeg"].first
+      a.add_extensions("sort-test")
+      assert_equal 2, [a, b].sort.length
+    end
+
     it "handles newline characters correctly" do
       assert_includes MIME::Types.type_for("test.pdf\n.txt"), "text/plain"
       assert_includes MIME::Types.type_for("test.txt\n.pdf"), "application/pdf"