Could not load file or assembly 'EPPlus, Version=4.0.5.0

This C# applications use EPPlus.
As it is, it works fine. I can export a DataGridView to an Excel file like so:

ExcelPackage package = new ExcelPackage();
using ((package)) // use EPPlus
{
    ExcelWorksheet worksheet = package.Workbook.Worksheets.Add(pageTitle);
    DataTable dataTable = GetDataTableFromDataGridView(dataGridView);
    worksheet.Cells["A1"].LoadFromDataTable(dataTable, true);
    worksheet.View.FreezePanes(2, 1); // freeze first row
    worksheet.Row(1).Style.Font.Bold = true;
    worksheet.Cells[worksheet.Dimension.Address].AutoFitColumns();
    FileUtilities.IsFileOpen(ExcelFile);                                           

    package.SaveAs(ExcelFile); // exception if file is open in excel               

    string message = null;                                                         
    message = "Open file?";                                                        
    string title = null;                                                           
    title = pathAndFile + " created";                                              

    DialogResult result = MessageBox.Show(message, title, MessageBoxButtons.YesNo);
    if (result == DialogResult.No)                                                 
    {                                                                              
        // just close message box                                                  
    }                                                                              
    else if (result == DialogResult.Yes)                                           
    {                                                                              
        System.Diagnostics.Process.Start(pathAndFile); // open excel file          
    }

}

However, if I make any change to this code, it causes an exception to be thrown. Even if I just add this line:

int x = 0;

it causes the exception:

System.IO.FileLoadException
HResult=0x80131040
Message=Could not load file or assembly 'EPPlus, Version=4.0.5.0, Culture=neutral, PublicKeyToken=ea159fdaa78159a1' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

If I look in the Project References, I see an EPPlus entry:

Aliasas          : global
Copy Local       : True
File Type        : Assembly
Runtime Version  : v4.0.30319

If I remove the new line, the exception does not occur and the code executes normally, i.e. it exports to the Excel file.

What is going wrong?