diff --git a/library/aes.c b/library/aes.c
index e48a2a6..f6dc996 100644
--- a/library/aes.c
+++ b/library/aes.c
@@ -1183,6 +1183,7 @@
     AES_VALIDATE_RET( ctx != NULL );
     AES_VALIDATE_RET( mode == MBEDTLS_AES_ENCRYPT ||
                       mode == MBEDTLS_AES_DECRYPT );
+    AES_VALIDATE_RET( data_unit != NULL );
     AES_VALIDATE_RET( input != NULL );
     AES_VALIDATE_RET( output != NULL );