diff --git a/lib/rack/test/utils.rb b/lib/rack/test/utils.rb
index d1f7456..fb4b859 100644
--- a/lib/rack/test/utils.rb
+++ b/lib/rack/test/utils.rb
@@ -105,7 +105,7 @@ def build_parts(buffer, parameters)
 
       # Append each multipart parameter value to the buffer.
       def _build_parts(buffer, parameters)
-        parameters.map do |name, value|
+        parameters.each do |name, value|
           if name =~ /\[\]\Z/ && value.is_a?(Array) && value.all? { |v| v.is_a?(Hash) }
             value.each do |hash|
               new_value = {}
@@ -113,7 +113,7 @@ def _build_parts(buffer, parameters)
               _build_parts(buffer, new_value)
             end
           else
-            [value].flatten.map do |v|
+            [value].flatten.each do |v|
               if v.respond_to?(:original_filename)
                 build_file_part(buffer, name, v)
               else