diff --git a/tests/grib_indexing.sh b/tests/grib_indexing.sh
index 453208f62..ee356393d 100755
--- a/tests/grib_indexing.sh
+++ b/tests/grib_indexing.sh
@@ -13,6 +13,7 @@
 label="grib_indexing_test"
 temp=temp.$label.index.out
 infile=${data_dir}/index.grib
+sample1=$ECCODES_SAMPLES_PATH/GRIB1.tmpl
 
 # Writes to "out.gribidx"
 $EXEC ${test_dir}/grib_indexing ${infile} > $temp
@@ -109,9 +110,32 @@ ${tools_dir}/grib_index_build -N -o $tempIndex2 $tempGribFile2
 ${tools_dir}/grib_compare -v $tempIndex1 $tempIndex2
 rm -f $tempIndex1 $tempIndex2 $tempGribFile1 $tempGribFile2
 
+# Indexes containing different keys
+${tools_dir}/grib_index_build -k date,time -N -o $tempIndex1 $sample1
+${tools_dir}/grib_index_build -k level     -N -o $tempIndex2 $sample1
+set +e
+${tools_dir}/grib_compare -v $tempIndex1 $tempIndex2 2>$tempOut
+status=$?
+set -e
+[ $status -ne 0 ]
+cat $tempOut
+grep -q "Indexes contained in the input files have different keys" $tempOut
+rm -f $tempIndex1 $tempIndex2 $tempOut
+
+
+${tools_dir}/grib_index_build -k date,time       -N -o $tempIndex1 $sample1
+${tools_dir}/grib_index_build -k date,time,level -N -o $tempIndex2 $sample1
+set +e
+${tools_dir}/grib_compare -v $tempIndex2 $tempIndex1 2>$tempOut
+status=$?
+set -e
+[ $status -ne 0 ]
+cat $tempOut
+grep -q "Indexes contained in the input files have different keys" $tempOut
+rm -f $tempIndex1 $tempIndex2 $tempOut
+
 # ECC-1516
 # ---------
-sample1=$ECCODES_SAMPLES_PATH/GRIB1.tmpl
 ${tools_dir}/grib_index_build -N -o $tempIndex1 $sample1 > /dev/null
 ${tools_dir}/grib_dump $tempIndex1 >/dev/null
 
diff --git a/tools/grib_tools.cc b/tools/grib_tools.cc
index ce038d07f..72122fbd1 100644
--- a/tools/grib_tools.cc
+++ b/tools/grib_tools.cc
@@ -511,17 +511,17 @@ static int grib_tool_index(grib_runtime_options* options)
             k2 = k2->next;
         }
         if (!found) {
-            printf("Indexes contained in the input files have different keys\n");
-            printf("keys in file %s:\n", f1);
+            fprintf(stderr, "Indexes contained in the input files have different keys!\n");
+            fprintf(stderr, "keys in file %s:\n", f1);
             k1 = options->index1->keys;
             while (k1) {
-                printf("\t%s\n", k1->name);
+                fprintf(stderr, "\t%s\n", k1->name);
                 k1 = k1->next;
             }
-            printf("keys in file %s:\n", f2);
+            fprintf(stderr, "keys in file %s:\n", f2);
             k2 = options->index2->keys;
             while (k2) {
-                printf("\t%s\n", k2->name);
+                fprintf(stderr, "\t%s\n", k2->name);
                 k2 = k2->next;
             }
             exit(1);
@@ -543,17 +543,17 @@ static int grib_tool_index(grib_runtime_options* options)
             k1 = k1->next;
         }
         if (!found) {
-            printf("Indexes contained in the input files have different keys\n");
-            printf("keys in file %s:\n", f2);
+            fprintf(stderr,"Indexes contained in the input files have different keys!\n");
+            fprintf(stderr, "keys in file %s:\n", f2);
             k2 = options->index2->keys;
             while (k2) {
-                printf("\t%s\n", k2->name);
+                fprintf(stderr, "\t%s\n", k2->name);
                 k2 = k2->next;
             }
-            printf("keys in file %s:\n", f1);
+            fprintf(stderr, "keys in file %s:\n", f1);
             k1 = options->index1->keys;
             while (k1) {
-                printf("\t%s\n", k1->name);
+                fprintf(stderr, "\t%s\n", k1->name);
                 k1 = k1->next;
             }