diff --git a/src/lib.rs b/src/lib.rs
index e9f3c25..e6e1464 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -99,7 +99,7 @@ pub trait Integer: private::Sealed {}
 
 // Seal to prevent downstream implementations of the Integer trait.
 mod private {
-    pub trait Sealed {
+    pub trait Sealed: Copy {
         fn write(self, buf: &mut crate::Buffer) -> &str;
     }
 }