diff --git a/src/ctap2.rs b/src/ctap2.rs
index 6cef808..88c4a82 100644
--- a/src/ctap2.rs
+++ b/src/ctap2.rs
@@ -948,6 +948,11 @@ impl<UP: UserPresence, T: TrussedRequirements> Authenticator for crate::Authenti
 
         // 6. process any options present
 
+        // RK is not supported in get_assertion
+        if parameters.options.as_ref().and_then(|options| options.rk).is_some() {
+            return Err(Error::InvalidOption);
+        }
+
         // UP occurs by default, but option could specify not to.
         let do_up = if parameters.options.is_some() {
             parameters.options.as_ref().unwrap().up.unwrap_or(true)