diff --git a/applypatch/applypatch.h b/applypatch/applypatch.h
index ee54c24..edec848 100644
--- a/applypatch/applypatch.h
+++ b/applypatch/applypatch.h
@@ -40,7 +40,7 @@
 // and use it as the source instead.
 #define CACHE_TEMP_SOURCE "/cache/saved.file"
 
-typedef ssize_t (*SinkFn)(unsigned char*, ssize_t, void*);
+typedef ssize_t (*SinkFn)(const unsigned char*, ssize_t, void*);
 
 // applypatch.c
 int ShowLicenses();