That is very likely because wdl_fft is expecting different sized data. Be sure to set WDL_FFT_REALSIZE=4 or 8 in your project preprocessor defines (must be projectwide so both sides see the same define). Set it to 4 if you are passing floats, 8 if you are passing doubles.
Also be sure you are calling wdl_fft_init() at some point before trying to calculate anything.
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