diff --git a/src/sound/backend/i_sound.h b/src/sound/backend/i_sound.h
index 83f5fc2c6..40b8a1425 100644
--- a/src/sound/backend/i_sound.h
+++ b/src/sound/backend/i_sound.h
@@ -36,6 +36,7 @@
 #define __I_SOUND__
 
 #include "i_soundinternal.h"
+#include "utility/zstring.h"
 
 class FileReader;
 struct FSoundChan;