diff --git a/lib/sass/script/parser.rb b/lib/sass/script/parser.rb
index 311c1ef7..49edecd4 100644
--- a/lib/sass/script/parser.rb
+++ b/lib/sass/script/parser.rb
@@ -66,10 +66,14 @@ RUBY
       def funcall
         return paren unless name = try_tok(:ident)
         # An identifier without arguments is just a string
-        return Script::String.new(name.last) unless try_tok(:lparen)
-        args = arglist || []
-        assert_tok(:rparen)
-        Script::Funcall.new(name.last, args)
+        unless try_tok(:lparen)
+          warn %Q{WARNING: Implicit strings are deprecated. '#{name.last}' was not quoted. Please add double quotes. E.g. "#{name.last}".}
+          Script::String.new(name.last)
+        else
+          args = arglist || []
+          assert_tok(:rparen)
+          Script::Funcall.new(name.last, args)
+        end
       end
 
       def arglist
diff --git a/test/sass/engine_test.rb b/test/sass/engine_test.rb
index ce0b7ef6..b1467ed5 100755
--- a/test/sass/engine_test.rb
+++ b/test/sass/engine_test.rb
@@ -719,7 +719,9 @@ SASS
   end
 
   def test_inaccessible_functions
-    assert_equal("a {\n  b: send(to_s); }\n", render("a\n  b = send(to_s)"))
+    assert_warning %Q{WARNING: Implicit strings are deprecated. 'to_s' was not quoted. Please add double quotes. E.g. \"to_s\".} do
+      assert_equal("a {\n  b: send(to_s); }\n", render("a\n  b = send(to_s)"))
+    end
     assert_equal("a {\n  b: public_instance_methods(); }\n", render("a\n  b = public_instance_methods()"))
   end