diff --git a/src/bitmap.c b/src/bitmap.c
index ebbeb8a..3e62db9 100644
--- a/src/bitmap.c
+++ b/src/bitmap.c
@@ -55,7 +55,7 @@ bitmap_compare(const unsigned char *a,
 }
 
 int
-bitmap_diffmap(const unsigned char *diff,
+bitmap_diffmap(unsigned char *diff,
                const unsigned char *a,
                const unsigned char *b)
 {
diff --git a/src/bitmap.h b/src/bitmap.h
index 1365755..7969b7f 100644
--- a/src/bitmap.h
+++ b/src/bitmap.h
@@ -11,7 +11,7 @@ bitmap_compare(const unsigned char *a,
                const unsigned char *b);
 
 int
-bitmap_diffmap(const unsigned char *diff,
+bitmap_diffmap(unsigned char *diff,
                const unsigned char *a,
-               const unsigned char *b)
+               const unsigned char *b);
 #endif