diff --git a/boilerplate/cairo-boilerplate-pdf.c b/boilerplate/cairo-boilerplate-pdf.c
index 7e5e16244..c3289f8fb 100644
--- a/boilerplate/cairo-boilerplate-pdf.c
+++ b/boilerplate/cairo-boilerplate-pdf.c
@@ -36,7 +36,7 @@
 #include <signal.h>
 #endif
 
-cairo_user_data_key_t pdf_closure_key;
+static const cairo_user_data_key_t pdf_closure_key;
 
 typedef struct _pdf_target_closure
 {