diff --git a/ext/dl/extconf.rb b/ext/dl/extconf.rb
index ef9f09438d..0eabff608d 100644
--- a/ext/dl/extconf.rb
+++ b/ext/dl/extconf.rb
@@ -7,7 +7,7 @@ end
 $INSTALLFILES = [
   ["dl.h", "$(archdir)$(target_prefix)", ""],
 ]
-$distcleanfiles = "callback.h"
+$distcleanfiles << "callback.h"
 
 
 check = true
diff --git a/ext/socket/extconf.rb b/ext/socket/extconf.rb
index 00821ccb01..da498639ad 100644
--- a/ext/socket/extconf.rb
+++ b/ext/socket/extconf.rb
@@ -268,7 +268,7 @@ end
 have_header("sys/un.h")
 have_header("sys/uio.h")
 
-$distcleanfiles = "constants.h"
+$distcleanfiles << "constants.h"
 
 if have_func(test_func)
   have_func("hsterror")