diff --git a/base85.c b/base85.c
index a9e97f89d..a6c41d597 100644
--- a/base85.c
+++ b/base85.c
@@ -37,7 +37,7 @@ static void prep_base85(void)
 	}
 }
 
-int decode_85(char *dst, char *buffer, int len)
+int decode_85(char *dst, const char *buffer, int len)
 {
 	prep_base85();
 
@@ -82,7 +82,7 @@ int decode_85(char *dst, char *buffer, int len)
 	return 0;
 }
 
-void encode_85(char *buf, unsigned char *data, int bytes)
+void encode_85(char *buf, const unsigned char *data, int bytes)
 {
 	prep_base85();
 
diff --git a/cache.h b/cache.h
index 384b26022..1904131f2 100644
--- a/cache.h
+++ b/cache.h
@@ -468,8 +468,8 @@ extern int pager_in_use;
 extern int pager_use_color;
 
 /* base85 */
-int decode_85(char *dst, char *line, int linelen);
-void encode_85(char *buf, unsigned char *data, int bytes);
+int decode_85(char *dst, const char *line, int linelen);
+void encode_85(char *buf, const unsigned char *data, int bytes);
 
 /* alloc.c */
 struct blob;