Invalid Column Name - Column Does Exist - sql-server

Question

I have a table to which I added a new column, as such:
ALTER TABLE [Invoice].[Invoice] ADD [DaysToPay] int
GO
ALTER TABLE [Invoice].[Invoice] ADD CONSTRAINT [DF_Invoice_DaysToPay] DEFAULT ((30)) FOR [DaysToPay]
GO
UPDATE [Invoice].[Invoice] SET [DaysToPay] = 30
GO
ALTER TABLE [Invoice].[Invoice] ALTER COLUMN [DaysToPay] int NOT NULL
GO
If I now do SELECT * FROM [Invoice].[Invoice] I get an Invalid column name 'DaysToPay' error. I get this error whether I run the statement through SSMS or any other means (via code).
The same error occurs with or without the column when I select specific columns. e.g. SELECT [InvoiceID] FROM [Invoice].[Invoice].
I have closed SSMS and restarted the SQL service. Still the same error.
The column was added under the same user that is getting the error and that same user is the owner of the schema.
Both these statements return 0 rows:
SELECT *
FROM INFORMATION_SCHEMA.COLUMNS
WHERE COLUMN_NAME = 'DaysToPay'
SELECT *
FROM sys.columns
WHERE Name = N'DaysToPay'
I've tried Ctrl + Shift + R but I don't think it's a SSMS thing and it made no difference.
The database is not replicated.
I ran a DBCC OPENTRAN are there were none active.
I have since removed the column (manually using the designer in SSMS) and I still get the same error. The table shows correctly in the SSMS designer (as it did prior to me manually removing it).
Any ideas other than dropping and re-creating the table?
EDIT:
The rest of the script that was run is as follows (I have removed certain columns so as not to identify):
ALTER PROCEDURE [Invoice].[Invoice_Save] (
#InvoiceID int,
#TypeID int,
#InvoiceNumber int,
#InvoiceDate datetime,
#DatePaid datetime,
#PaymentEnteredBy int,
#AmountPaid float,
#PaymentTypeID int,
#HasVAT bit,
#VATRate float,
#TranslationPercentageCharge float,
#IsVoid bit,
#VoidDate datetime,
#VoidBy int,
#DateSent datetime,
#SentBy int,
#PurchaseOrderNumber varchar(100),
#BillingContact varchar(255),
#BillingAddress varchar(MAX),
#BillingContactEmail nvarchar(255),
#BillingContactTelNo varchar(50),
#Notes varchar(MAX),
#CompanyID int,
#DaysToPay int,
#DateCreated datetime,
#CreatedBy int,
#LastUpdated datetime,
#LastUpdatedBy int
)
AS
IF NOT EXISTS (SELECT * FROM [Invoice].[Invoice] WHERE [InvoiceID] = #InvoiceID)
BEGIN
INSERT INTO [Invoice].[Invoice] ([TypeID], [InvoiceNumber], [InvoiceDate], , [DatePaid], [PaymentEnteredBy], [AmountPaid], [PaymentTypeID], [HasVAT], [VATRate], [TranslationPercentageCharge], [IsVoid], [VoidDate], [VoidBy], [DateSent], [SentBy], [PurchaseOrderNumber], [BillingContact], [BillingAddress], [BillingContactEmail], [BillingContactTelNo], [Notes], [CompanyID], [DaysToPay], [DateCreated], [CreatedBy], [LastUpdated], [LastUpdatedBy])
VALUES (#TypeID, #InvoiceNumber, #InvoiceDate, #DatePaid, #PaymentEnteredBy, #AmountPaid, #PaymentTypeID, #HasVAT, #VATRate, #TranslationPercentageCharge, #IsVoid, #VoidDate, #VoidBy, #DateSent, #SentBy, #PurchaseOrderNumber, #BillingContact, #BillingAddress, #BillingContactEmail, #BillingContactTelNo, #Notes, #CompanyID, #DaysToPay, #DateCreated, #CreatedBy, #LastUpdated, #LastUpdatedBy)
SELECT MAX([InvoiceID]) AS MaxID FROM [Invoice].[Invoice]
END
ELSE
UPDATE [Invoice].[Invoice]
SET [TypeID] = #TypeID,
[InvoiceNumber] = #InvoiceNumber,
[InvoiceDate] = #InvoiceDate,
[DatePaid] = #DatePaid,
[PaymentEnteredBy] = #PaymentEnteredBy,
[AmountPaid] = #AmountPaid,
[PaymentTypeID] = #PaymentTypeID,
[HasVAT] = #HasVAT,
[VATRate] = #VATRate,
[TranslationPercentageCharge] = #TranslationPercentageCharge,
[IsVoid] = #IsVoid,
[VoidDate] = #VoidDate,
[VoidBy] = #VoidBy,
[DateSent] = #DateSent,
[SentBy] = #SentBy,
[PurchaseOrderNumber] = #PurchaseOrderNumber,
[BillingContact] = #BillingContact,
[BillingAddress] = #BillingAddress,
[BillingContactEmail] = #BillingContactEmail,
[BillingContactTelNo] = #BillingContactTelNo,
[Notes] = #Notes,
[CompanyID] = #CompanyID,
[DaysToPay] = #DaysToPay,
[DateCreated] = #DateCreated,
[CreatedBy] = #CreatedBy,
[LastUpdated] = #LastUpdated,
[LastUpdatedBy] = #LastUpdatedBy
WHERE [InvoiceID] = #InvoiceID
GO