diff --git a/imagery/i.atcorr/computations.cpp b/imagery/i.atcorr/computations.cpp
index 53f8664ee06..6e5a38c5b07 100644
--- a/imagery/i.atcorr/computations.cpp
+++ b/imagery/i.atcorr/computations.cpp
@@ -102,14 +102,14 @@ double trunca()
     for (i = 0; i < 83; i++) {
         if (rmu[i] > 0.8)
             break;
-        k = i - 1;
+        k = i;
     }
 
     int kk = 0;
     for (i = 0; i < 83; i++) {
         if (rmu[i] > 0.94)
             break;
-        kk = i - 1;
+        kk = i;
     }
 
     double aa =
@@ -118,7 +118,7 @@ double trunca()
     double x1 = (double)(log10(sixs_trunc.pha[kk]));
     double x2 = (double)acos(rmu[kk]);
 
-    for (i = kk + 1; i < 83; i++) {
+    for (i = kk; i < 83; i++) {
         double a;
         if (fabs(rmu[i] - 1) <= 1e-08)
             a = x1 - aa * x2;
@@ -445,9 +445,14 @@ void os(const double tamoy, const double trmoy, const double pizmoy,
         /* compute position of the plane layer */
         double taup = tap + trp;
         iplane = -1;
-        for (int i = 0; i <= ntp; i++)
+        for (int i = 0; i <= ntp; i++) {
             if (taup >= h[i])
                 iplane = i;
+        }
+        if (iplane == -1) {
+            G_fatal_error(
+                _("Position of the plane layer could not be determined"));
+        }
 
         /* update the layer from the end to the position to update if necessary
          */
@@ -1006,9 +1011,14 @@ void iso(const double tamoy, const double trmoy, const double pizmoy,
         /* compute position of the plane layer */
         double taup = tap + trp;
         iplane = -1;
-        for (int i = 0; i <= ntp; i++)
+        for (int i = 0; i <= ntp; i++) {
             if (taup >= h[i])
                 iplane = i;
+        }
+        if (iplane == -1) {
+            G_fatal_error(
+                _("Position of the plane layer could not be determined"));
+        }
 
         /* update the layer from the end to the position to update if necessary
          */