diff --git a/library/aes.c b/library/aes.c
index 4afc3c4..d2b05e2 100644
--- a/library/aes.c
+++ b/library/aes.c
@@ -971,6 +971,7 @@
                            unsigned char output[16] )
 {
     AES_VALIDATE_RET( ctx != NULL );
+    AES_VALIDATE_RET( ctx->rk != NULL );
     AES_VALIDATE_RET( input != NULL );
     AES_VALIDATE_RET( output != NULL );
     AES_VALIDATE_RET( mode == MBEDTLS_AES_ENCRYPT ||