diff --git a/mrbgems/mruby-bin-mirb/tools/mirb/mirb_completion.c b/mrbgems/mruby-bin-mirb/tools/mirb/mirb_completion.c
index 58f117733..06ca3317d 100644
--- a/mrbgems/mruby-bin-mirb/tools/mirb/mirb_completion.c
+++ b/mrbgems/mruby-bin-mirb/tools/mirb/mirb_completion.c
@@ -22,6 +22,10 @@
 /* Windows compatibility */
 #ifdef _MSC_VER
 #define strdup _strdup
+#endif
+
+/* strndup is not available on Windows (MSVC and MinGW) */
+#ifdef _WIN32
 static char*
 strndup(const char *s, size_t n)
 {