diff --git a/include/psa/crypto_builtin_primitives.h b/include/psa/crypto_builtin_primitives.h
index 75801a1..8701eb7 100644
--- a/include/psa/crypto_builtin_primitives.h
+++ b/include/psa/crypto_builtin_primitives.h
@@ -113,7 +113,10 @@
     psa_algorithm_t alg;
     uint8_t iv_length;
     uint8_t block_length;
-    mbedtls_cipher_context_t cipher;
+    union {
+        unsigned int initialised;
+        mbedtls_cipher_context_t cipher;
+    } ctx;
 } mbedtls_psa_cipher_operation_t;
 
 #define MBEDTLS_PSA_CIPHER_OPERATION_INIT {0, 0, 0, {0}}