diff --git a/lib/addressable/template.rb b/lib/addressable/template.rb
index bc520415..68506947 100644
--- a/lib/addressable/template.rb
+++ b/lib/addressable/template.rb
@@ -922,6 +922,7 @@ def normalize_value(value)
     # @return [Hash]
     #   A hash with stringified keys
     def normalize_keys(mapping)
+      return mapping unless mapping.respond_to?(:inject)
       return mapping.inject({}) do |accu, pair|
         name, value = pair
         if Symbol === name
diff --git a/spec/addressable/template_spec.rb b/spec/addressable/template_spec.rb
index 24616c23..df3743dd 100644
--- a/spec/addressable/template_spec.rb
+++ b/spec/addressable/template_spec.rb
@@ -102,6 +102,12 @@
   }
 end
 
+describe "Substituting from an optional block" do
+  sub = lambda { |x| "foo-#{x}" }
+
+  Addressable::Template.new("{bar}/{baz}").expand(sub).to_str.should == "foo-bar/foo-baz"
+end
+
 describe "Level 1:" do
   subject {
     {:var => "value", :hello => "Hello World!"}