diff --git a/ext/openssl/ossl.c b/ext/openssl/ossl.c
index a3b612b..b75796e 100644
--- a/ext/openssl/ossl.c
+++ b/ext/openssl/ossl.c
@@ -92,7 +92,7 @@ ossl_x509_ary2sk(VALUE ary)
 
 #define OSSL_IMPL_SK2ARY(name, type)	        \
 VALUE						\
-ossl_##name##_sk2ary(STACK *sk)			\
+ossl_##name##_sk2ary(STACK_OF(type) *sk)			\
 {						\
     type *t;					\
     int i, num;					\
diff --git a/ext/openssl/ossl_pkcs7.c b/ext/openssl/ossl_pkcs7.c
index f7cd156..aec0023 100644
--- a/ext/openssl/ossl_pkcs7.c
+++ b/ext/openssl/ossl_pkcs7.c
@@ -572,7 +572,7 @@ ossl_pkcs7_add_certificate(VALUE self, VALUE cert)
     return self;
 }
 
-static STACK *
+static void *
 pkcs7_get_certs_or_crls(VALUE self, int want_certs)
 {
     PKCS7 *pkcs7;
diff --git a/ext/openssl/ossl_ssl.c b/ext/openssl/ossl_ssl.c
index c278edc..e74e891 100644
--- a/ext/openssl/ossl_ssl.c
+++ b/ext/openssl/ossl_ssl.c
@@ -623,10 +623,10 @@ ossl_sslctx_get_ciphers(VALUE self)
     if (!ciphers)
         return rb_ary_new();
 
-    num = sk_num((STACK*)ciphers);
+    num = sk_num((STACK_OF(SSL_CIPHER)*)ciphers);
     ary = rb_ary_new2(num);
     for(i = 0; i < num; i++){
-        cipher = (SSL_CIPHER*)sk_value((STACK*)ciphers, i);
+        cipher = (SSL_CIPHER*)sk_value((STACK_OF(SSL_CIPHER)*)ciphers, i);
         rb_ary_push(ary, ossl_ssl_cipher_to_ary(cipher));
     }
     return ary;