A student determines the molar concentration of \(\text{Cu}^{2+}\text{(aq)}\) in an industrial wastewater sample using spectrophotometry. A calibration curve of absorbance versus \([\text{Cu}^{2+}]\) is generated at \(\lambda_{\text{max}} = 620\text{ nm}\) using standard solutions of \(\text{Cu(NO}_3\text{)}_2\text{(aq)}\) dissolved in distilled water, with the spectrophotometer properly zeroed using a distilled water blank. The wastewater sample contains \(\text{Cu}^{2+}\text{(aq)}\) along with a nonreactive organic additive that also absorbs light at \(620\text{ nm}\). If the student measures the absorbance of the wastewater sample directly without zeroing the spectrophotometer with a blank containing the organic additive, what is the effect on the calculated \([\text{Cu}^{2+}]\) in the sample, and why?