diff --git a/src/cairo-pdf-surface-private.h b/src/cairo-pdf-surface-private.h
index 5c70ef973..11e026bb9 100644
--- a/src/cairo-pdf-surface-private.h
+++ b/src/cairo-pdf-surface-private.h
@@ -67,7 +67,7 @@ typedef struct _cairo_pdf_group_resources {
 typedef struct _cairo_pdf_source_surface_entry {
     cairo_hash_entry_t base;
     unsigned int id;
-    const unsigned char *unique_id;
+    unsigned char *unique_id;
     unsigned long unique_id_length;
     cairo_bool_t interpolate;
     cairo_pdf_resource_t surface_res;
diff --git a/src/cairo-pdf-surface.c b/src/cairo-pdf-surface.c
index 35315c1f9..e0321a9fb 100644
--- a/src/cairo-pdf-surface.c
+++ b/src/cairo-pdf-surface.c
@@ -1177,7 +1177,8 @@ _cairo_pdf_surface_add_source_surface (cairo_pdf_surface_t	*surface,
     surface_key.id  = source->unique_id;
     surface_key.interpolate = interpolate;
     cairo_surface_get_mime_data (source, CAIRO_MIME_TYPE_UNIQUE_ID,
-				 &surface_key.unique_id, &surface_key.unique_id_length);
+				 (const unsigned char **) &surface_key.unique_id,
+				 &surface_key.unique_id_length);
     _cairo_pdf_source_surface_init_key (&surface_key);
     surface_entry = _cairo_hash_table_lookup (surface->all_surfaces, &surface_key.base);
     if (surface_entry) {