diff --git a/Modules/tkappinit.c b/Modules/tkappinit.c
index 42b6bb88304..1c676325ece 100644
--- a/Modules/tkappinit.c
+++ b/Modules/tkappinit.c
@@ -12,6 +12,7 @@
    it explicitly, e.g. tkapp.eval("load {} Blt").
  */
 
+#include <string.h>
 #include <tcl.h>
 #include <tk.h>