diff --git a/Library/Homebrew/formula.rb b/Library/Homebrew/formula.rb
index d0f6a03890..c6e5aae51b 100644
--- a/Library/Homebrew/formula.rb
+++ b/Library/Homebrew/formula.rb
@@ -590,8 +590,8 @@ EOF
         p = {:filename => '%03d-homebrew.diff' % n+=1, :compression => false}
 
         if defined? DATA and url == DATA
-          pn=Pathname.new p[:filename]
-          pn.write DATA.read
+          pn = Pathname.new p[:filename]
+          pn.write(DATA.read.to_s.gsub("HOMEBREW_PREFIX", HOMEBREW_PREFIX))
         elsif url =~ %r[^\w+\://]
           out_fn = p[:filename]
           case url