diff --git a/src/grib_accessor_class_unsigned.c b/src/grib_accessor_class_unsigned.c
index d1e09634e..093d2a500 100644
--- a/src/grib_accessor_class_unsigned.c
+++ b/src/grib_accessor_class_unsigned.c
@@ -235,14 +235,14 @@ int pack_long_unsigned_helper(grib_accessor* a, const long* val, size_t* len, in
 
         /* Check if value fits into number of bits */
         if (check) {
-            const long nbits = self->nbytes * 8;
+            if (v < 0) {
+                grib_context_log(a->context, GRIB_LOG_ERROR,
+                                "Key \"%s\": Trying to encode a negative value of %ld for key of type unsigned\n", a->name, v);
+                return GRIB_ENCODING_ERROR;
+            }
             /* See GRIB-23 and GRIB-262 */
             if (!value_is_missing(v)) {
-                if (v < 0) {
-                    grib_context_log(a->context, GRIB_LOG_ERROR,
-                                     "Key \"%s\": Trying to encode a negative value of %ld for key of type unsigned\n", a->name, v);
-                    return GRIB_ENCODING_ERROR;
-                }
+                const long nbits = self->nbytes * 8;
                 if (nbits < 33) {
                     unsigned long maxval = (1UL << nbits) - 1;
                     if (maxval > 0 && v > maxval) { /* See ECC-1002 */
diff --git a/tests/grib_set.sh b/tests/grib_set.sh
index 325d4168e..2871f661d 100755
--- a/tests/grib_set.sh
+++ b/tests/grib_set.sh
@@ -13,7 +13,8 @@
 REDIRECT=/dev/null
 
 infile=${data_dir}/regular_gaussian_surface.grib1
-outfile=${data_dir}/set.grib1
+outfile=${data_dir}/temp.grib_set.grib
+temp=temp.grib_set.out
 
 rm -f $outfile
 
@@ -52,13 +53,37 @@ levtype=`${tools_dir}/grib_get -p levtype:l $outfile`
 centre=`${tools_dir}/grib_get -p centre:l $outfile`
 [ $centre -eq 80 ]
 
+# Set without -s. Expected to fail
+# ----------------------------------------------------
 set +e
-# This is expected to fail
 ${tools_dir}/grib_set -p levtype $infile $outfile 2> $REDIRECT > $REDIRECT
-[ $? -ne 0 ]
+status=$?
 set -e
+[ $status -ne 0 ]
 
-# GRIB-941: encoding of grib2 angles
+# Out-of-bounds value. Expected to fail
+# ----------------------------------------------------
+input=${data_dir}/reduced_gaussian_sub_area.grib2
+set +e
+${tools_dir}/grib_set -s perturbationNumber=1000 $input $outfile 2>$temp
+status=$?
+set -e
+[ $status -ne 0 ]
+grep -q "Trying to encode value of 1000 but the maximum allowable value is 255 (number of bits=8)" $temp
+
+# Negative value for an unsigned key. Expected to fail
+# ----------------------------------------------------
+input=${data_dir}/reduced_gaussian_sub_area.grib2
+set +e
+${tools_dir}/grib_set -s perturbationNumber=-1 $input $outfile 2>$temp
+status=$?
+set -e
+[ $status -ne 0 ]
+grep -q "Trying to encode a negative value of -1 for key of type unsigned" $temp
+
+
+# GRIB-941: encoding of GRIB2 angles
+# -----------------------------------
 angleInDegrees=130.9989
 angleInMicroDegrees=130998900
 files="GRIB2.tmpl regular_gg_pl_grib2.tmpl reduced_gg_pl_320_grib2.tmpl polar_stereographic_pl_grib2.tmpl"
@@ -69,14 +94,17 @@ for f in $files; do
 done
 
 # GRIB-943: centre code table
+# ----------------------------
 ${tools_dir}/grib_set -s centre=289 $ECCODES_SAMPLES_PATH/GRIB2.tmpl $outfile
 ${tools_dir}/grib_dump -O $outfile | grep -q 'centre = 289.*Zambia'
 
 # ECC-539: avoid output being the same as input
+# -----------------------------------------------
 set +e
 ${tools_dir}/grib_set -s centre=0 $outfile $outfile
 status=$?
 set -e
 [ $status -ne 0 ]
 
-rm -f $outfile
+
+rm -f $outfile $temp