Complex square root of -1 : zsqrt(-1)

Hi,
With gforth, complex.fs included.
When calculating the square root of -1, I find that:
-1e 0e 0.5e 0e z** z. 0.0000000000000000612303176911189+1.i  ok
-1e 0e zsqrt z. NaN ok
What's the problem with zsqrt?
NB.
-1e 1e-16 zsqrt z. 0.0000000000000000612303176911189+1.i  ok
-1e 1e-100 zsqrt z. 0.0000000000000000612303176911189+1.i  ok
Ahmed