diff --git a/src/test/run-pass/issue-20971.rs b/src/test/run-fail/issue-20971.rs
similarity index 93%
rename from src/test/run-pass/issue-20971.rs
rename to src/test/run-fail/issue-20971.rs
index e854cce4446bd..818f0e1394196 100644
--- a/src/test/run-pass/issue-20971.rs
+++ b/src/test/run-fail/issue-20971.rs
@@ -10,6 +10,8 @@
 
 // Regression test for Issue #20971.
 
+// error-pattern:Hello, world!
+
 pub trait Parser {
     type Input;
     fn parse(&mut self, input: <Self as Parser>::Input);
@@ -23,7 +25,7 @@ impl Parser for () {
 }
 
 pub fn many() -> Box<Parser<Input=<() as Parser>::Input> + 'static> {
-    panic!()
+    panic!("Hello, world!")
 }
 
 fn main() {