diff --git a/lib/rouge/lexer.rb b/lib/rouge/lexer.rb
index 089d030ec8..c5dd7fcd74 100644
--- a/lib/rouge/lexer.rb
+++ b/lib/rouge/lexer.rb
@@ -213,7 +213,7 @@ def detectable?
       # @private
       def register(name, lexer)
         # reset an existing list of lexers
-        @all = nil if @all
+        @all = nil if defined?(@all)
         registry[name.to_s] = lexer
       end