diff --git a/openmls/src/group/mls_group/membership.rs b/openmls/src/group/mls_group/membership.rs
index af1a38596..14cc67ba7 100644
--- a/openmls/src/group/mls_group/membership.rs
+++ b/openmls/src/group/mls_group/membership.rs
@@ -14,6 +14,8 @@ use crate::{
     storage::OpenMlsProvider, treesync::LeafNode,
 };
 
+#[cfg_attr(feature = "async", maybe_async::must_be_async)]
+#[cfg_attr(not(feature = "async"), maybe_async::must_be_sync)]
 impl MlsGroup {
     /// Adds members to the group.
     ///
diff --git a/openmls/src/group/mls_group/proposal.rs b/openmls/src/group/mls_group/proposal.rs
index 09cf15e76..b5b68a7d4 100644
--- a/openmls/src/group/mls_group/proposal.rs
+++ b/openmls/src/group/mls_group/proposal.rs
@@ -64,6 +64,8 @@ macro_rules! impl_propose_fun {
         /// Creates proposals to add an external PSK to the key schedule.
         ///
         /// Returns an error if there is a pending commit.
+        #[cfg_attr(feature = "async", maybe_async::must_be_async)]
+        #[cfg_attr(not(feature = "async"), maybe_async::must_be_sync)]
         pub async fn $name<Provider: OpenMlsProvider>(
             &mut self,
             provider: &Provider,