diff --git a/hs20/client/est.c b/hs20/client/est.c
index c3f27e1e9..5c6e2f67d 100644
--- a/hs20/client/est.c
+++ b/hs20/client/est.c
@@ -218,7 +218,9 @@ typedef struct {
 	} d;
 } AttrOrOID;
 
+#if OPENSSL_VERSION_NUMBER >= 0x10100000L
 DEFINE_STACK_OF(AttrOrOID)
+#endif
 
 typedef struct {
 	int type;