diff --git a/src/libmbedcrypto.c b/src/libmbedcrypto.c
index 755c5ebe..a2e74d3b 100644
--- a/src/libmbedcrypto.c
+++ b/src/libmbedcrypto.c
@@ -687,7 +687,7 @@ static void cipher_encrypt(struct ssh_cipher_struct *cipher,
 }
 
 static void cipher_encrypt_cbc(struct ssh_cipher_struct *cipher, void *in, void *out,
-        unsigned long len)
+        size_t len)
 {
     size_t outlen = 0;
     int rc = 0;
@@ -745,7 +745,7 @@ static void cipher_decrypt(struct ssh_cipher_struct *cipher,
 }
 
 static void cipher_decrypt_cbc(struct ssh_cipher_struct *cipher, void *in, void *out,
-        unsigned long len)
+        size_t len)
 {
     size_t outlen = 0;
     int rc = 0;