尝试使用合并功能时出错：

Question

我是R的新手。在尝试运行这些代码非常简单的行时会出现因子级错误：

Attendance_1415 <- merge(Locations_1415, Absences_1415,by=c("Employee_ID","Date"), all.x=TRUE) 
Attendance_1415 <- merge(Attendance_1415, PayCodes_1415, by=c("Employee_ID","Date"), all.x=TRUE)