diff --git a/src/engine.rs b/src/engine.rs
index e734623..5645386 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -170,7 +170,7 @@ def_package! {
                 let has_signatory = participant_keys.iter().any(|p| signatories.contains(p));
                 if !has_signatory {
                     return Err(Box::new(rhai::EvalAltResult::ErrorRuntime(
-                        format!("Access denied: none of the participants are signatories").into(),
+                        format!("Access denied: none of the participants are signatories. Signatories: {}", signatories.join(", ")).into(),
                         context.position()
                     )));
                 }