diff --git a/include/mbedtls/x509_internal.h b/include/mbedtls/x509_internal.h
index 19d63ec..3816c36 100644
--- a/include/mbedtls/x509_internal.h
+++ b/include/mbedtls/x509_internal.h
@@ -33,9 +33,13 @@
 struct mbedtls_x509_crt;
 struct mbedtls_pk_context;
 struct mbedtls_x509_crt_frame;
+#define MBEDTLS_X509_CACHE_PK_READERS_MAX    ((uint32_t) -1)
+#define MBEDTLS_X509_CACHE_FRAME_READERS_MAX ((uint32_t) -1)
 typedef struct mbedtls_x509_crt_cache
 {
 #if defined(MBEDTLS_THREADING_C)
+    uint32_t frame_readers;
+    uint32_t pk_readers;
     mbedtls_threading_mutex_t frame_mutex;
     mbedtls_threading_mutex_t pk_mutex;
 #endif