Possible bug in Function Lookup syntax check in interface?

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In WinRATS 9.00 on Windows 7 x64, if I open the program, select View -> Standard Functions, select a function like %ACOS or %MAXINDEX, then click "Syntax Check" without entering anything in the input field, RATS displays the expected dialog bog ("SX16. Missing Operand or Adjacent Operators"). When I click OK on this dialog box, RATS crashes. Is this a bug?

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FYI, as of RATS 9.00g, this bug appears to be fixed in the sense that it no longer crashes RATS. For %ACOS (and maybe other functions), clicking OK on the first dialog box, which is where the bug appeared in the past, now displays a dialog that says "SX22. Expected type REAL, Got VECTOR[REAL] instead". This still seems strange, since I can't place why RATS is treating an empty argument as a vector of reals. Maybe this is a bug, maybe it isn't.