How to prevent the error: Arithmetic overflow error converting expression to data type nvarchar ?

I am developing an Access application with Accesss as the front end and SQL Server as the
backend database.

I have an existing table named tblMutualFunds comprised of the following fields:

MutualID int 4 (Identity)
State nvarchar (2)
Account nvarchar (9)

I also have an Excel file that one of my users created with the fields ResStateCode and Plan.

I use the TransferSpreadsheet command to Import the Excel file into an Access table named tblAppend.
My goal is to append the records from the imported Access table tblAppend into the table named tblMutualFunds.

I noticed that when the tblAppend table gets created, the fields generated are defined as follows:

ResStateCode  nvarchar(255)
[Plan #] float (8)

field ResStateCode of tblAppend correlates to field State on the table tblMutualFund
field [Plan #] of tblAppend correlates to field Account on the table tblMutualFund

When I try to perform the following

insert into dbo.tblMutualFunds
select ResStateCode, [Plan #]
from dbo.tblMuFunds

I get the error message:

Server: Msg 8115, Level 16, State 2, Line 27
Arithmetic overflow error converting expression to data type nvarchar.
The statement has been terminated.

The problem seems to be that the field [Plan #] gets converted from Excel into a Float field.
Now I am trying to insert values from table tblAppend for field Plan of type float
into a field Account of type nvarchar in table tblMutualFunds.

Do you know how I can fix this conversion problem ?
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I was going to try to Alter the table tblMuFunds and alter the column [Plan #] to type nvarchar but I get the following error:

alter table dbo.tblMuFunds
alter column [Plan #] nvarchar not null

Server: Msg 8115, Level 16, State 2, Line 1
Arithmetic overflow error converting expression to data type nvarchar.
The statement has been terminated.