diff --git a/c/external.c b/c/external.c
index fabeb36..0e8a5af 100644
--- a/c/external.c
+++ b/c/external.c
@@ -1114,7 +1114,7 @@ s48_value
 s48_enter_substring(char *str, int length)
 {
   s48_value obj = s48_allocate_stob(S48_STOBTYPE_STRING, length + 1);
-  strncpy(S48_UNSAFE_EXTRACT_STRING(obj), str, length);
+  memcpy(S48_UNSAFE_EXTRACT_STRING(obj), str, length);
   *(S48_UNSAFE_EXTRACT_STRING(obj) + length) = '\0';
   return obj;
 }