You get this error when code has been compiled in VFP 5. Since the function was unknown, VFP 5 generates a user defined function call with the name INDEXSEEK. VFP does not evaluate a function name at runtime to figure out if this is a native function or a user defined function. This happens exclusively at compile time once. Recompiling the code in VFP 6 or later will fix the problem.
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Christof