diff --git a/lib/thor/parser/arguments.rb b/lib/thor/parser/arguments.rb
index fc887ac51..05c1a659c 100644
--- a/lib/thor/parser/arguments.rb
+++ b/lib/thor/parser/arguments.rb
@@ -30,7 +30,11 @@ def initialize(arguments = [])
 
       arguments.each do |argument|
         if !argument.default.nil?
-          @assigns[argument.human_name] = argument.default
+          begin
+            @assigns[argument.human_name] = argument.default.dup
+          rescue TypeError  # Compatibility shim for un-dup-able Fixnum in Ruby < 2.4
+            @assigns[argument.human_name] = argument.default
+          end
         elsif argument.required?
           @non_assigned_required << argument
         end
diff --git a/spec/parser/options_spec.rb b/spec/parser/options_spec.rb
index 843119d78..12dbda36a 100644
--- a/spec/parser/options_spec.rb
+++ b/spec/parser/options_spec.rb
@@ -302,11 +302,10 @@ def remaining
             "Expected '--fruit' to be one of #{enum.join(', ')}; got orange")
       end
 
-      it "allows multiple values if repeatable is specified" do
-        create :foo => Thor::Option.new("foo", :type => :string, :repeatable => true)
-
+      it "does not erroneously mutate defaults" do
+        create :foo => Thor::Option.new("foo", :type => :string, :repeatable => true, :required => false, :default => [])
         expect(parse("--foo=bar", "--foo", "12")["foo"]).to eq(["bar", "12"])
-        expect(parse("--foo", "13", "--foo", "14")["foo"]).to eq(["bar", "12", "13", "14"])
+        expect(@opt.instance_variable_get(:@switches)["--foo"].default).to eq([])
       end
     end