diff --git a/compiler/rustc_expand/src/mbe/macro_rules.rs b/compiler/rustc_expand/src/mbe/macro_rules.rs
index a074af0189a..66463eeb907 100644
--- a/compiler/rustc_expand/src/mbe/macro_rules.rs
+++ b/compiler/rustc_expand/src/mbe/macro_rules.rs
@@ -1173,7 +1173,8 @@ fn quoted_tt_to_string(tt: &mbe::TokenTree) -> String {
         mbe::TokenTree::MetaVar(_, name) => format!("${}", name),
         mbe::TokenTree::MetaVarDecl(_, name, kind) => format!("${}:{}", name, kind),
         _ => panic!(
-            "unexpected mbe::TokenTree::{{Sequence or Delimited}} \
+            "{}",
+            "unexpected mbe::TokenTree::{Sequence or Delimited} \
              in follow set checker"
         ),
     }