Compiler error

[loewal]

I have been searching for 8266 esp-now examples.
Sorry, but not ONE was without errors...missing pieces of code, errors compiling for Lolin Node mcu V3, etc.
In your slave: line 38
C:\Users\lwalt\OneDrive\Arduino\Simple_esp_now_8266_slave\Simple_esp_now_8266_slave.ino: In function 'void setup()':

Simple_esp_now_8266_slave:38:38: error: invalid conversion from 'void ()(const uint8_t, const uint8_t*, int) {aka void ()(const unsigned char, const unsigned char*, int)}' to 'esp_now_recv_cb_t {aka void ()(unsigned char, unsigned char*, unsigned char)}' [-fpermissive]

esp_now_register_recv_cb(OnDataRecv);

                                  ^

In file included from C:\Users\lwalt\OneDrive\Arduino\Simple_esp_now_8266_slave\Simple_esp_now_8266_slave.ino:4:0:

C:\Users\lwalt\AppData\Local\Arduino15\packages\esp8266\hardware\esp8266\2.5.2/tools/sdk/include/espnow.h:50:5: error: initializing argument 1 of 'int esp_now_register_recv_cb(esp_now_recv_cb_t)' [-fpermissive]

int esp_now_register_recv_cb(esp_now_recv_cb_t cb);

 ^

exit status 1
invalid conversion from 'void ()(const uint8_t, const uint8_t*, int) {aka void ()(const unsigned char, const unsigned char*, int)}' to 'esp_now_recv_cb_t {aka void ()(unsigned char, unsigned char*, unsigned char)}' [-fpermissive]

Please,C:\Users\lwalt\OneDrive\Arduino\Simple_esp_now_8266_slave\Simple_esp_now_8266_slave.ino: In function 'void setup()':

Simple_esp_now_8266_slave:38:38: error: invalid conversion from 'void ()(const uint8_t, const uint8_t*, int) {aka void ()(const unsigned char, const unsigned char*, int)}' to 'esp_now_recv_cb_t {aka void ()(unsigned char, unsigned char*, unsigned char)}' [-fpermissive]

esp_now_register_recv_cb(OnDataRecv);

                                  ^

In file included from C:\Users\lwalt\OneDrive\Arduino\Simple_esp_now_8266_slave\Simple_esp_now_8266_slave.ino:4:0:

C:\Users\lwalt\AppData\Local\Arduino15\packages\esp8266\hardware\esp8266\2.5.2/tools/sdk/include/espnow.h:50:5: error: initializing argument 1 of 'int esp_now_register_recv_cb(esp_now_recv_cb_t)' [-fpermissive]

int esp_now_register_recv_cb(esp_now_recv_cb_t cb);

 ^

exit status 1
invalid conversion from 'void ()(const uint8_t, const uint8_t*, int) {aka void ()(const unsigned char, const unsigned char*, int)}' to 'esp_now_recv_cb_t {aka void ()(unsigned char, unsigned char*, unsigned char)}' [-fpermissive]

Please, please Help