This isn't a question, more of an FYI. While trying to import an agreement record on step 3 of the wizard there were no warning or critical errors on the record's cells, including the Master agreement number, however when I get to step 5 I received this message.
After working with the file a bit I remembered some issues with Excel formatting and spaces throwing mysterious error messages. Although I couldn't see it when looking at the file , when I'd copied my value from somewhere I'd inadvertently acquired a space  after the Master agreement number.
When I removed the space the record was imported just fine.
On a previous occasion, I had to copy the data to Notepad to strip out all Excel formatting, close Excel and start with a fresh worksheet, to get rid of whatever was lurking in the proverbial light spectrum I couldn't see.