diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index 3e09260cf..d6bafc064 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -1790,7 +1790,7 @@ hb_buffer_add_codepoints (hb_buffer_t          *buffer,
  **/
 HB_EXTERN void
 hb_buffer_append (hb_buffer_t *buffer,
-		  hb_buffer_t *source,
+		  const hb_buffer_t *source,
 		  unsigned int start,
 		  unsigned int end)
 {
diff --git a/src/hb-buffer.h b/src/hb-buffer.h
index 001e6dca8..5df17170d 100644
--- a/src/hb-buffer.h
+++ b/src/hb-buffer.h
@@ -522,7 +522,7 @@ hb_buffer_add_codepoints (hb_buffer_t          *buffer,
 
 HB_EXTERN void
 hb_buffer_append (hb_buffer_t *buffer,
-		  hb_buffer_t *source,
+		  const hb_buffer_t *source,
 		  unsigned int start,
 		  unsigned int end);