diff --git a/reader/read-suppress.lsp b/reader/read-suppress.lsp
index e00a78cf44757329e342ef964426dbdc7d84d7be..a8eccd5bdf1f05d82be870429349d41adae9d513 100644
--- a/reader/read-suppress.lsp
+++ b/reader/read-suppress.lsp
@@ -27,7 +27,8 @@
 (def-read-suppress-test read-suppress.12 "(...)")
 
 (defparameter *non-macro-chars*
-  "1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-=+_~!@$%^&*{}[]<>/?.")
+  (coerce "1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-=+_~!@$%^&*{}[]<>/?."
+          'simple-base-string))
 
 (declaim (type simple-base-string *non-macro-chars*))