Examples of theoretical fits to selected FeH lines and resulting distributions of filling factors for two spectra of GJ 1002 taken in two different observing runs with CRIRES. Top panels: run 385.D-0273, T_eff = 3100 K, α(Fe) = −4.37, υsini = 2.43 km s^-1. Middle panels: run 079.D-0357, T_eff = 3100 K, υsini = 1.85 km s^-1, α(Fe) = −4.47. Bottom panels: same as top panels but adopting atmospheric parameters from the middle panels: implementation of these parameters results in a spurious detection of the magnetic field as illustrated on the corresponding plot of filling factors (see text). Dashed violet line: computation with zero magnetic field; red line: computation with multi-component magnetic field shown on the right hand side; blue line: computations with homogeneous magnetic field (i.e. f = 1) of the same average intensity as multi-component magnetic field. Vertical dashed lines mark the edges of the actual fit region.