diff --git a/lib/appraisal/conditional.rb b/lib/appraisal/conditional.rb
index db15ff33..ad2a8e1a 100644
--- a/lib/appraisal/conditional.rb
+++ b/lib/appraisal/conditional.rb
@@ -16,6 +16,8 @@ def to_s
 
     # :nodoc:
     def for_dup
+      return unless @condition.is_a?(String)
+
       "install_if #{@condition} do\n#{indent(super)}\nend"
     end
   end