division by zero mystery

As a hobby project, I am writing a C library of functions that do operations on complex numbers. I am also porting this code to Visual Basic (v.6) in the form of a calculator. I'm seeing something very odd. Code that generates a division by zero error in VB seems to work correctly in the C routines. For example, it is sometimes necessary to evaluate:

x = atan(b / a) (where a and b are the real and imag parts of a complex number)

This generates a division by zero error in VB when the complex operand is, for example, 0.0 + 1.0i but seems to work correctly in the C routines. That is, I can calculate i ^ i correctly with the C code but get a division by zero error when atan() is evaluated in VB. Any thoughts on why this is happening? Is it just "lucky" undefined behaviour?

JS
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