diff --git a/crates/tinymcp/src/error/mod.rs b/crates/tinymcp/src/error/mod.rs
index ac431894..11e47810 100644
--- a/crates/tinymcp/src/error/mod.rs
+++ b/crates/tinymcp/src/error/mod.rs
@@ -403,6 +403,10 @@ impl Error {
     }
 
     /// Builds a [`Self::InvalidArgument`] from anything printable.
+    ///
+    /// Only the `TinyBus` adapter (the `module` feature) raises this error, so the
+    /// constructor does not exist in a build without it.
+    #[cfg(any(feature = "module", test))]
     pub(crate) fn invalid_argument(detail: impl std::fmt::Display) -> Self {
         Self::InvalidArgument {
             detail: detail.to_string(),