diff --git a/src/lib.rs b/src/lib.rs
index 0d11a84d8..fa8a98416 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -938,7 +938,7 @@ impl EGraph {
         run_report: &mut RunReport,
         search_results: &mut HashMap<Symbol, SearchResult>,
     ) {
-        let rules = self.rulesets.get(&ruleset).unwrap();
+        let rules = self.rulesets.get(&ruleset).unwrap_or_else(|| panic!("Unknown ruleset {}", ruleset));
         match rules {
             Ruleset::Rules(_ruleset_name, rule_names) => {
                 let copy_rules = rule_names.clone();