Within QGIS, I joined .dbf table containing population data to a shapefile of Census tracts. The join field is called GISJOIN in both the target table and the shapefile's table, and this attribute has the "string" data type in both tables. When I look at the attribute table of the population table after it is loaded within QGIS, all the data is there and looks fine. When I compare the values of the GISJOIN fields in each table to each other, they match up perfectly.
HOWEVER when I perform the join, inside the Properties dialog, and then look at the shapefile's attribute table, all the new attribute headings are there, added in, but all the values are "NULL."
Any idea what is going on here?