diff --git a/m29f400bt.c b/m29f400bt.c
index 8f0329e..c2236e5 100644
--- a/m29f400bt.c
+++ b/m29f400bt.c
@@ -28,8 +28,7 @@
    functions. */
 
 /* chunksize is 1 */
-int write_m29f400bt(struct flashctx *flash, uint8_t *src, unsigned int start,
-		    unsigned int len)
+int write_m29f400bt(struct flashctx *flash, const uint8_t *src, unsigned int start, unsigned int len)
 {
 	int i;
 	chipaddr bios = flash->virtual_memory;