diff --git a/examples/Cargo.toml b/examples/Cargo.toml
index fe7a397..8b1c7e7 100644
--- a/examples/Cargo.toml
+++ b/examples/Cargo.toml
@@ -21,7 +21,7 @@ name = "demo-server"
 path = "src/server.rs"
 
 [dependencies]
-tsp = { path = "../tsp" }
+tsp = { path = "../tsp", features = ["async"] }
 axum = { workspace = true}
 base64ct = { workspace = true }
 clap = { workspace = true}
diff --git a/tsp/src/error.rs b/tsp/src/error.rs
index 0157207..0c3a7ca 100644
--- a/tsp/src/error.rs
+++ b/tsp/src/error.rs
@@ -31,8 +31,12 @@ pub enum Error {
     MissingVid(String),
     #[error("Error: unresolved vid {0}")]
     UnverifiedVid(String),
+    #[cfg(feature = "async")]
     #[error("Error: no relation with sender {0}")]
     UnverifiedSource(String, Option<Vec<u8>>),
+    #[cfg(not(feature = "async"))]
+    #[error("Error: no relation with sender {0}")]
+    UnverifiedSource(String),
     #[error("Error: unresolved next hop {0}")]
     UnresolvedNextHop(String),
     #[error("Error: no relation with next hop {0}")]
diff --git a/tsp/src/store.rs b/tsp/src/store.rs
index 25788e6..0a4041c 100644
--- a/tsp/src/store.rs
+++ b/tsp/src/store.rs
@@ -544,7 +544,10 @@ impl Store {
                 let sender = String::from_utf8(sender.to_vec())?;
 
                 let Ok(sender_vid) = self.get_verified_vid(&sender) else {
+                    #[cfg(feature = "async")]
                     return Err(Error::UnverifiedSource(sender, None));
+                    #[cfg(not(feature = "async"))]
+                    return Err(Error::UnverifiedSource(sender));
                 };
 
                 let (nonconfidential_data, payload) =
@@ -565,6 +568,7 @@ impl Store {
                         if self.get_verified_vid(inner_vid).is_err() {
                             return Err(Error::UnverifiedSource(
                                 inner_vid.to_owned(),
+                                #[cfg(feature = "async")]
                                 Some(inner.to_vec()),
                             ));
                         }