diff --git a/src/lookahead.rs b/src/lookahead.rs
index 91d55f1..8ee1554 100644
--- a/src/lookahead.rs
+++ b/src/lookahead.rs
@@ -147,7 +147,7 @@
     type Token: Token;
 }
 
-impl<F: FnOnce(TokenMarker) -> T, T: Token> Peek for F {
+impl<F: Copy + FnOnce(TokenMarker) -> T, T: Token> Peek for F {
     type Token = T;
 }
 
@@ -165,6 +165,6 @@
 
 mod private {
     use super::{Token, TokenMarker};
-    pub trait Sealed {}
-    impl<F: FnOnce(TokenMarker) -> T, T: Token> Sealed for F {}
+    pub trait Sealed: Copy {}
+    impl<F: Copy + FnOnce(TokenMarker) -> T, T: Token> Sealed for F {}
 }