Insert Stored Procedure With Error Check And Transaction Function

Hi, guys
I try to add some error check and transaction and rollback function on my insert stored procedure but I have an error "Error converting data type varchar to smalldatatime" if i don't use /*error check*/ code, everything went well and insert a row into contract table.
could you correct my code, if you know what is the problem?

thanks

My contract table DDL:
************************************************** ***

create table contract(
contractNum int identity(1,1) primary key,
contractDate smalldatetime not null,
tuition money not null,
studentId char(4) not null foreign key references student (studentId),
contactId int not null foreign key references contact (contactId)
);


My insert stored procedure is:
************************************************** *****

create proc sp_insert_new_contract
( @contractDate[smalldatetime],
@tuition [money],
@studentId[char](4),
@contactId[int])
as

if not exists (select studentid
from student
where studentid = @studentId)
begin
print 'studentid is not a valid id'
return -1
end

if not exists (select contactId
from contact
where contactId = @contactId)
begin
print 'contactid is not a valid id'
return -1
end
begin transaction

insert into contract
([contractDate],
[tuition],
[studentId],
[contactId])
values
(@contractDate,
@tuition,
@studentId,
@contactId)

/*Error Check */
if @@error !=0 or @@rowcount !=1
begin
rollback transaction
print ‘Insert is failed’
return -1
end
print ’New contract has been added’

commit transaction
return 0
go