SearchByDistance on Joined Table

[CollinGrove2]

I joined a .dbf attribute table to a File GDB Feature Class, and want to do a spatial search. It works just fine on the FGDB Feature Class, by itself, without joining. However, when I join a table to it, and do a SearchByDistance, I get the error:

Details: Cannot project to null coordinate system
Parameter name: newCoordinateSystem

Anybody have any ideas? The error definitely comes from the SearchByDistance method...

[AndreiIvanov]

Your joined table is likely without projection. Set projection, and it should be fine.

[CollinGrove2]

Your joined table is likely without projection. Set projection, and it should be fine.

What am I setting the projection to? It's just a spreadsheet table that I've saved to a .dbf. Do I need to just set it to the same projection as the FGDB Feature Class?