diff --git a/src/libs/decoders/SDL_sound.c b/src/libs/decoders/SDL_sound.c
index 25f4cc1f..61397a56 100644
--- a/src/libs/decoders/SDL_sound.c
+++ b/src/libs/decoders/SDL_sound.c
@@ -29,6 +29,7 @@
  */
 
 #include <stdio.h>
+#include <ctype.h>
 
 #if HAVE_CONFIG_H
 #  include <config.h>