https://core.tcl-lang.org/expect/info/094d670f6250f34c

 exp_clib.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/exp_clib.c b/exp_clib.c
index 172c05e..c86dda4 100644
--- a/exp_clib.c
+++ b/exp_clib.c
@@ -1476,8 +1476,8 @@ expDiagLogU(str)
      char *str;
 {
   if (exp_is_debugging) {
-    fprintf(stderr,str);
-    if (exp_logfile) fprintf(exp_logfile,str);
+    fprintf(stderr,"%s", str);
+    if (exp_logfile) fprintf(exp_logfile,"%s", str);
   }
 }