diff.patch

diff --git a/lib/rack/utils.rb b/lib/rack/utils.rb
index 0c7091e..30dc81d 100644
--- a/lib/rack/utils.rb
+++ b/lib/rack/utils.rb
@@ -158,11 +158,11 @@ module Rack
         value.map { |k, v|
           build_nested_query(v, prefix ? "#{prefix}[#{escape(k)}]" : escape(k))
         }.join("&")
-      when String
+      when nil
+        prefix
+      else
         raise ArgumentError, "value must be a Hash" if prefix.nil?
         "#{prefix}=#{escape(value)}"
-      else
-        prefix
       end
     end
     module_function :build_nested_query
diff --git a/test/spec_utils.rb b/test/spec_utils.rb
index 622b8ff..0bb228e 100644
--- a/test/spec_utils.rb
+++ b/test/spec_utils.rb
@@ -221,6 +221,8 @@ describe Rack::Utils do

     Rack::Utils.build_nested_query("foo" => "1", "bar" => "2").
       should.be equal_query_to("foo=1&bar=2")
+    Rack::Utils.build_nested_query("foo" => 1, "bar" => 2).
+      should.be equal_query_to("foo=1&bar=2")
     Rack::Utils.build_nested_query("my weird field" => "q1!2\"'w$5&7/z8)?").
       should.be equal_query_to("my+weird+field=q1%212%22%27w%245%267%2Fz8%29%3F")