In Subtitle Workshop, trying to use the Join tool to join two SAMI files and recalculate the timings would result in this error every time:

The following error (EConvertError) was derived from object TfrmJoin (frmJoin): 

"" is not a valid floating point value"

The solution is: open each of the original SAMI files in turn and make sure to give them each a value for "input FPS" and "FPS". Resave each. Then run the Join tool. No more error.