Strange behaviour in GRC for a simple FFT/IFFT flow-graph

Hi all!
When constructing a simple flow graph
Signal_source(f)->Throttle(f)->Stream_to_vector(vf)->Forward_FFT(vc)->Reverse_FFT(vc)->Vector_to_stream©->WX_GUI_FFT_sink©,
the sink exhibits strange behaviour, showing frequency (and the image
frequency) different than the one I’m creating with the signal source,
e.g. for f=1kHz, sink shows 15kHz and -15kHz whereas for f=15 kHZ it
shows 1kHz and -1kHz.
On the other hand, if I create a complex signal using signal source, or
use Float_to_complex block before feeding it to Stream_to_vector, the
results are as expected.
Is this a glitch in GR (I’m using GRC 3.6.4.1.), or is there something
I’m missing here?
The flow graph and the result for f=1kHz are as
follows:http://imageshack.us/photo/my-images/194/xfvr.jpg/http://imageshack.us/photo/my-images/826/dj2p.jpg/
*f=float; vc=vector of complex; c=complex
Thanks in advance!

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