diff --git a/src/rukka_lang/mod.rs b/src/rukka_lang/mod.rs
index c42d79c..334f65b 100644
--- a/src/rukka_lang/mod.rs
+++ b/src/rukka_lang/mod.rs
@@ -154,7 +154,7 @@ impl EvaluatorState {
         },
         _ => return Err(format!("Bad if expression"))
       },
-      _ => unimplemented!(),
+      s => return Err(format!("Non-existent special form {}; this should never happen", s)),
     })
   }