See https://lists.gnu.org/archive/html/bug-mailutils/2010-10/msg00005.html

diff -u mailutils-2.2/libmu_scm/mu_message.c mailutils-2.2.new/libmu_scm/mu_message.c
--- mailutils-2.2/libmu_scm/mu_message.c	2010-04-18 10:29:07.000000000 -0700
+++ mailutils-2.2.new/libmu_scm/mu_message.c	2016-07-03 21:18:53.746185547 -0700
@@ -510,8 +510,8 @@
     {
       SCM car = SCM_CAR (lst);
       if (scm_is_string (car)
 	  && mu_c_strncasecmp (scm_i_string_chars (car), name,
-			       scm_i_string_length (car)) == 0)
+			       scm_c_string_length (car)) == 0)
 	return 1;
     }
   return 0;