diff --git a/src/core/buffer.c b/src/core/buffer.c
index 48588b6e..b300d4ec 100644
--- a/src/core/buffer.c
+++ b/src/core/buffer.c
@@ -109,7 +109,7 @@ static inline void unfree_block ( struct buffer *buffer,
  *
  * Returns 1 for success, 0 for failure (e.g. buffer too small).
  */
-int fill_buffer ( struct buffer *buffer, void *data,
+int fill_buffer ( struct buffer *buffer, const void *data,
 		  off_t offset, size_t len ) {
 	struct buffer_free_block desc;
 	physaddr_t block, prev_block;
diff --git a/src/include/buffer.h b/src/include/buffer.h
index a22b00dd..0085c7d1 100644
--- a/src/include/buffer.h
+++ b/src/include/buffer.h
@@ -39,7 +39,7 @@ extern struct buffer load_buffer;
 /* Functions in buffer.c */
 
 extern void init_buffer ( struct buffer *buffer );
-extern int fill_buffer ( struct buffer *buffer, void *data,
+extern int fill_buffer ( struct buffer *buffer, const void *data,
 			 off_t offset, size_t len );
 
 #endif /* BUFFER_H */