diff --git a/flash.h b/flash.h
index 24861ba..da049d1 100644
--- a/flash.h
+++ b/flash.h
@@ -260,7 +260,7 @@
 #ifdef __MINGW32__
 char* strtok_r(char *str, const char *delim, char **nextp);
 #endif
-#if defined(__DJGPP__)
+#if defined(__DJGPP__) || !defined(HAVE_STRNLEN)
 size_t strnlen(const char *str, size_t n);
 #endif