diff --git a/preprocess.c b/preprocess.c
index dbbddd65..3b08e731 100644
--- a/preprocess.c
+++ b/preprocess.c
@@ -445,7 +445,10 @@ void process_grdecl(const struct grdecl   *in,
 	  double z1 = sign*in->zcorn[i+2*nx*(j+2*ny*(k))];
 	  double z2 = sign*in->zcorn[i+2*nx*(j+2*ny*(k+1))];
 	  
-	  if (z2 < z1){
+          int c1 = i/2 + nx*(j/2 + ny*k/2);
+          int c2 = i/2 + nx*(j/2 + ny*(k+1)/2);
+
+          if (in->actnum[c1] && in->actnum[c2] && (z2 < z1)){
 	    fprintf(stderr, "\nZCORN should be strictly nondecreasing along pillars!\n");	 
 	    error = 1;
 	    goto end;