diff --git a/library/aes.c b/library/aes.c
index e27e40a..4f3351f 100644
--- a/library/aes.c
+++ b/library/aes.c
@@ -1397,7 +1397,8 @@
 #if defined(MBEDTLS_CIPHER_MODE_CBC)
     unsigned char prv[16];
 #endif
-#if defined(MBEDTLS_CIPHER_MODE_CTR) || defined(MBEDTLS_CIPHER_MODE_CFB)
+#if defined(MBEDTLS_CIPHER_MODE_CTR) || defined(MBEDTLS_CIPHER_MODE_CFB) || \
+    defined(MBEDTLS_CIPHER_MODE_OFB)
     size_t offset;
 #endif
 #if defined(MBEDTLS_CIPHER_MODE_CTR)