diff --git a/lib/net/imap.rb b/lib/net/imap.rb
index 18868b47..0b383738 100644
--- a/lib/net/imap.rb
+++ b/lib/net/imap.rb
@@ -3901,6 +3901,7 @@ def fetch_internal(cmd, set, attr, mod = nil, partial: nil, changedsince: nil)
         raise ArgumentError, "partial can only be used with uid_fetch"
       end
       set = SequenceSet[set]
+      mod = mod.dup if mod && (partial || changedsince)
       if partial
         mod ||= []
         mod << "PARTIAL" << PartialRange[partial]