diff --git a/imagery/i.atcorr/computations.cpp b/imagery/i.atcorr/computations.cpp
index 6e5a38c5b07..aa2a61fa9c5 100644
--- a/imagery/i.atcorr/computations.cpp
+++ b/imagery/i.atcorr/computations.cpp
@@ -111,7 +111,8 @@ double trunca()
             break;
         kk = i;
     }
-
+    if (kk < 0)
+        kk = 0;
     double aa =
         (double)((log10(sixs_trunc.pha[kk]) - log10(sixs_trunc.pha[k])) /
                  (acos(rmu[kk]) - acos(rmu[k])));