C Question - '=': value truncated

Why does the following code produce a C192: '=': value truncated error? They are all uint8_t's. Structure definition shown below. What is there to truncate, all variables are of the exact same type! The result is correct but I am just interested in why.

msg.DataBuffer[7] = msg.DataBuffer[0] ^ msg.DataBuffer[1] ^ msg.DataBuffer[6];

typedef struct
{ uint16_t CANID; uint8_t DataBuffer[8]; uint8_t DataSize;
} CAN_MESSAGE;