diff --git a/libfreefare/freefare.h b/libfreefare/freefare.h
index 5083e21..23b66df 100644
--- a/libfreefare/freefare.h
+++ b/libfreefare/freefare.h
@@ -370,8 +370,8 @@ int		 mifare_desfire_delete_file (MifareTag tag, uint8_t file_no);
 
 ssize_t		 mifare_desfire_read_data (MifareTag tag, uint8_t file_no, off_t offset, size_t length, void *data);
 ssize_t		 mifare_desfire_read_data_ex (MifareTag tag, uint8_t file_no, off_t offset, size_t length, void *data, int cs);
-ssize_t		 mifare_desfire_write_data (MifareTag tag, uint8_t file_no, off_t offset, size_t length, void *data);
-ssize_t		 mifare_desfire_write_data_ex (MifareTag tag, uint8_t file_no, off_t offset, size_t length, void *data, int cs);
+ssize_t		 mifare_desfire_write_data (MifareTag tag, uint8_t file_no, off_t offset, size_t length, const void *data);
+ssize_t		 mifare_desfire_write_data_ex (MifareTag tag, uint8_t file_no, off_t offset, size_t length, const void *data, int cs);
 int		 mifare_desfire_get_value (MifareTag tag, uint8_t file_no, int32_t *value);
 int		 mifare_desfire_get_value_ex (MifareTag tag, uint8_t file_no, int32_t *value, int cs);
 int		 mifare_desfire_credit (MifareTag tag, uint8_t file_no, int32_t amount);
diff --git a/libfreefare/mifare_desfire.c b/libfreefare/mifare_desfire.c
index f4cc537..24c5df0 100644
--- a/libfreefare/mifare_desfire.c
+++ b/libfreefare/mifare_desfire.c
@@ -1626,13 +1626,13 @@ write_data (MifareTag tag, uint8_t command, uint8_t file_no, off_t offset, size_
 }
 
 ssize_t
-mifare_desfire_write_data (MifareTag tag, uint8_t file_no, off_t offset, size_t length, void *data)
+mifare_desfire_write_data (MifareTag tag, uint8_t file_no, off_t offset, size_t length, const void *data)
 {
     return mifare_desfire_write_data_ex (tag, file_no, offset, length, data, madame_soleil_get_write_communication_settings (tag, file_no));
 }
 
 ssize_t
-mifare_desfire_write_data_ex (MifareTag tag, uint8_t file_no, off_t offset, size_t length, void *data, int cs)
+mifare_desfire_write_data_ex (MifareTag tag, uint8_t file_no, off_t offset, size_t length, const void *data, int cs)
 {
     return write_data (tag, 0x3D, file_no, offset, length, data, cs);
 }