diff --git a/src/sodium/secure_buffer.cr b/src/sodium/secure_buffer.cr
index 05713a3..a7d4248 100644
--- a/src/sodium/secure_buffer.cr
+++ b/src/sodium/secure_buffer.cr
@@ -49,7 +49,10 @@ module Sodium
     # For .dup
     def initialize(sbuf : self)
       initialize sbuf.bytesize
-      sbuf.to_slice.copy_to self.to_slice
+      # Maybe not thread safe
+      sbuf.readonly do
+        sbuf.to_slice.copy_to self.to_slice
+      end
       readonly
     end