diff --git a/indep.c b/indep.c
index 055e19e..4d13a3f 100644
--- a/indep.c
+++ b/indep.c
@@ -821,7 +821,7 @@ growbuf_reserve(struct growbuf *gb, int leastarea)
 }
 
 void
-growbuf_append(struct growbuf *gb, const char *src, int len)
+growbuf_append(struct growbuf *gb, const unsigned char *src, int len)
 {
     growbuf_reserve(gb, gb->length + len);
     memcpy(&gb->ptr[gb->length], src, len);
diff --git a/indep.h b/indep.h
index 5c6b1da..b123c85 100644
--- a/indep.h
+++ b/indep.h
@@ -83,7 +83,7 @@ extern void growbuf_init_without_GC(struct growbuf *gb);
 extern void growbuf_clear(struct growbuf *gb);
 extern Str growbuf_to_Str(struct growbuf *gb);
 extern void growbuf_reserve(struct growbuf *gb, int leastarea);
-extern void growbuf_append(struct growbuf *gb, const char *src, int len);
+extern void growbuf_append(struct growbuf *gb, const unsigned char *src, int len);
 #define GROWBUF_ADD_CHAR(gb,ch) ((((gb)->length>=(gb)->area_size)?growbuf_reserve(gb,(gb)->length+1):(void)0),(void)((gb)->ptr[(gb)->length++] = (ch)))
 
 extern char *w3m_auxbin_dir();