diff --git a/accepted/engine-host-functions.md b/accepted/engine-host-functions.md
index 303bf69..196a712 100644
--- a/accepted/engine-host-functions.md
+++ b/accepted/engine-host-functions.md
@@ -101,7 +101,7 @@ pub fn define_function(
     module: &str,
     name: &str,
     ty: FuncType,
-    func: impl Fn(Caller<'_>, &[Val], &mut [Val]) -> Result<(), Trap> + 'static,
+    func: impl Fn(Caller<'_>, &[Val], &mut [Val]) -> Result<(), Trap> + Send + Sync + 'static,
 ) -> Result<()>;
 
 /// Defines a function for the `Engine` from the given Rust closure.
@@ -113,7 +113,7 @@ pub fn wrap_function<Params, Results>(
     &self,
     module: &str,
     name: &str,
-    func: impl IntoFunc<Params, Results>,
+    func: impl IntoFunc<Params, Results> + Send + Sync,
 ) -> Result<()>;
 ```