I will present one of the latest results of our GTC exoplanet transit spectroscopy survey. Over the last years our group has obtained ground-based optical (538 nm - 918 nm) spectrophotometric transit observations for several hot Jupiters including HAT-P-32Ab using the OSIRIS (Optical System for Imaging and low Resolution Integrated Spectroscopy) instrument at the Spanish 10-meter telescope GTC.I will discuss the source, nature and proper correction of instrument specific systematic noise we found to affect our data. After its correction, we were able to yield high quality results with a precision between 482 and 1703 ppm depending on the wavelength channel. We measure a flat optical transmission spectrum for HAT-P-32Ab, consistent with the results of Gibson et al. (2013, MNRAS, 436, 2974) obtained with GMOS at Gemini-North. This independent reproduction of consistent results re-establishes faith in the reliability of ground-based transmission spectroscopy and emphasizes the high potential of OSIRIS at the GTC as a tool to complement current and future space-based observations.