diff --git a/include/polarssl/certs.h b/include/polarssl/certs.h
index aa5a6c3..b982d7e 100644
--- a/include/polarssl/certs.h
+++ b/include/polarssl/certs.h
@@ -27,14 +27,13 @@
 extern "C" {
 #endif
 
-extern char test_ca_crt[];
-extern char test_ca_key[];
-extern char test_ca_pwd[];
-extern char test_srv_crt[];
-extern char test_srv_key[];
-extern char test_cli_crt[];
-extern char test_cli_key[];
-extern char xyssl_ca_crt[];
+extern const char test_ca_crt[];
+extern const char test_ca_key[];
+extern const char test_ca_pwd[];
+extern const char test_srv_crt[];
+extern const char test_srv_key[];
+extern const char test_cli_crt[];
+extern const char test_cli_key[];
 
 #ifdef __cplusplus
 }