diff --git a/lib/.gitignore b/lib/.gitignore
index db09928b..7f9045d4 100644
--- a/lib/.gitignore
+++ b/lib/.gitignore
@@ -313,6 +313,7 @@
 /stdlib.h
 /stdlib.in.h
 /stpcpy.c
+/stpncpy.c
 /strchrnul.c
 /strchrnul.valgrind
 /strdup.c
diff --git a/m4/.gitignore b/m4/.gitignore
index 5389544d..cca78f48 100644
--- a/m4/.gitignore
+++ b/m4/.gitignore
@@ -175,6 +175,7 @@
 /stdio_h.m4
 /stdlib_h.m4
 /stpcpy.m4
+/stpncpy.m4
 /strchrnul.m4
 /strdup.m4
 /strerror.m4
diff --git a/src/glyphs.c b/src/glyphs.c
index 0c0e9fc2..b69659d8 100644
--- a/src/glyphs.c
+++ b/src/glyphs.c
@@ -63,8 +63,8 @@ static long
 on_success (const char *buf, size_t buflen, void *callback_arg)
 {
   callback_arg_t *arg = (callback_arg_t *) callback_arg;
-  assert (buflen < sizeof arg->buf);
-  strncpy (arg->buf, buf, buflen);
+  assert (buflen + 1 < sizeof arg->buf);
+  *stpncpy (arg->buf, buf, buflen) = '\0';
   return 1;
 }
 
@@ -73,7 +73,7 @@ on_failure (unsigned code MAYBE_UNUSED, const char *msg MAYBE_UNUSED,
             void *callback_arg)
 {
   callback_arg_t *arg = (callback_arg_t *) callback_arg;
-  assert (strlen (arg->fallback) < sizeof arg->buf);
+  assert (strlen (arg->fallback) + 1 < sizeof arg->buf);
   strcpy (arg->buf, arg->fallback);
   return 0;
 }