diff --git a/collects/mzlib/plt-match.ss b/collects/mzlib/plt-match.ss
index a862a6d..1d6fb60 100644
--- a/collects/mzlib/plt-match.ss
+++ b/collects/mzlib/plt-match.ss
@@ -173,13 +173,13 @@
       ((val)
        (raise
         (make-exn:misc:match
-         (format "match: no matching clause for ~e" val)
+         (string->immutable-string (format "match: no matching clause for ~e" val))
          (current-continuation-marks)
          val)))
       ((val expr)
        (raise
         (make-exn:misc:match
-         (format "match: no matching clause for ~e: ~s" val expr)
+         (string->immutable-string (format "match: no matching clause for ~e: ~s" val expr))
          (current-continuation-marks)
          val)))))