#0  0x00007fb92905cff6 in strlen () from /lib64/libc.so.6
#1  0x00007fb92905cd0e in strdup () from /lib64/libc.so.6
#2  0x00000000004185b9 in xstrdup (ptr=ptr@entry=0x0) at misc.c:259
#3  0x000000000042356c in define_variable_in_set (name=name@entry=0x428e23 "MAKE_TERMOUT",
    length=length@entry=12, value=0x0, origin=origin@entry=o_default, recursive=recursive@entry=0,
    set=set@entry=0x635fc0 <global_variable_set>, flocp=0x0) at variable.c:243
#4  0x0000000000406f29 in main (argc=<optimized out>, argv=<optimized out>, envp=0x7ffeaacd8da8)
    at main.c:1404

diff --git a/main.c b/main.c
index b2d169c..0cdb8a8 100644
--- a/main.c
+++ b/main.c
@@ -1429,13 +1429,18 @@ main (int argc, char **argv, char **envp)
 #ifdef HAVE_ISATTY
     if (isatty (fileno (stdout)))
       if (! lookup_variable (STRING_SIZE_TUPLE ("MAKE_TERMOUT")))
-        define_variable_cname ("MAKE_TERMOUT", TTYNAME (fileno (stdout)),
-                               o_default, 0)->export = v_export;
-
+        {
+          const char *tty = TTYNAME (fileno (stdout));
+          define_variable_cname ("MAKE_TERMOUT", tty ? tty : DEFAULT_TTYNAME,
+                                 o_default, 0)->export = v_export;
+        }
     if (isatty (fileno (stderr)))
       if (! lookup_variable (STRING_SIZE_TUPLE ("MAKE_TERMERR")))
-        define_variable_cname ("MAKE_TERMERR", TTYNAME (fileno (stderr)),
-                               o_default, 0)->export = v_export;
+        {
+          const char *tty = TTYNAME (fileno (stderr));
+          define_variable_cname ("MAKE_TERMERR", tty ? tty : DEFAULT_TTYNAME,
+                                 o_default, 0)->export = v_export;
+        }
 #endif
 
   /* Reset in case the switches changed our minds.  */
diff --git a/makeint.h b/makeint.h
index 6223936..2009f41 100644
--- a/makeint.h
+++ b/makeint.h
@@ -436,10 +436,11 @@ extern struct rlimit stack_limit;
 /* The number of bytes needed to represent the largest integer as a string.  */
 #define INTSTR_LENGTH         CSTRLEN ("18446744073709551616")
 
+#define DEFAULT_TTYNAME "true"
 #ifdef HAVE_TTYNAME
 # define TTYNAME(_f) ttyname (_f)
 #else
-# define TTYNAME(_f) "true"
+# define TTYNAME(_f) DEFAULT_TTYNAME
 #endif