diff --git a/s1process.py b/s1process.py
index 362f2989f70c6e44f48bb63e65a5b82c80893c34..3352a3ef76ba96b82b2ad19cd82aea126303c8a6 100644
--- a/s1process.py
+++ b/s1process.py
@@ -495,7 +495,7 @@ def extractROI(infiles, outdir, roi, roiname=None, ncores=12, overwrite=False, v
             """
             mbm = otb.Registry.CreateApplication('BandMathX')
             mbm.SetParameterStringList('il', l)
-            mbm.SetParameterString('exp', 'vmax({' + ';'.join(['im%db1' % i for i in len(l)]) + '})')
+            mbm.SetParameterString('exp', 'vmax({' + ';'.join(['im%db1' % i for i in range(len(l))]) + '})')
             mbm.SetParameterString('out', outfile)
             mbm.ExecuteAndWriteOutput()