diff --git a/Formula/rdiff-backup.rb b/Formula/rdiff-backup.rb
index daeeef88907..452a28d97d7 100644
--- a/Formula/rdiff-backup.rb
+++ b/Formula/rdiff-backup.rb
@@ -12,7 +12,7 @@ class RdiffBackup < Formula
     # We remove 'ppc' support, so we can pass Intel-optimized CFLAGS.
     archs = archs_for_command("python")
     archs.remove_ppc!
-    archs.delete! :x86_64 if Hardware.is_32_bit?
+    archs.delete :x86_64 if Hardware.is_32_bit?
     ENV['ARCHFLAGS'] = archs.as_arch_flags
 
     system "python", "setup.py", "--librsync-dir=#{prefix}", "build"