Exchange rate in expense

When loading credit card transactions, 4 USD lines and one CAD line, from My Wallet into an expense sheet, the CAD line has exchange rate of 0, whereas the USD all have different rates. The business unit's base currency is CAD and the employee is Canadian who went to the US for a conference. The one CAD line was for a meal while in Canada before flying to US.

When the "check expense for errors button" in the expense detail page was clicked, the error message at the top displays "Exchange Rate - Exchange rate must be 1 when expense currency is same as your base currency." The exchange rate field is red and greyed out.