diff --git a/nifti2/nifti_regress_test/cmake_testscripts/c22_copy_image.sh b/nifti2/nifti_regress_test/cmake_testscripts/c22_copy_image.sh
index a3abddc2..18c435c9 100644
--- a/nifti2/nifti_regress_test/cmake_testscripts/c22_copy_image.sh
+++ b/nifti2/nifti_regress_test/cmake_testscripts/c22_copy_image.sh
@@ -11,6 +11,23 @@ DATA=$2
 OUT_DATA=$(dirname ${DATA}) #Need to write to separate directory
 cd ${OUT_DATA}
 
+
+# Compare two NIfTI files by content rather than by compressed bytes.
+#
+# gzip output is not reproducible across zlib implementations: zlib-ng,
+# which Arch, CachyOS and other current distributions ship as the system
+# zlib, encodes the same input differently from stock zlib.  Comparing
+# the .gz files directly therefore fails on those systems even though the
+# image data round-trips perfectly.  Decompress first and compare that.
+nii_cmp() {
+    if [ "${1##*.}" = "gz" ]; then
+        gzip -dc "$1" > "$1.raw" && gzip -dc "$2" > "$2.raw" || return 1
+        cmp "$1.raw" "$2.raw"
+        return $?
+    fi
+    cmp "$1" "$2"
+}
+
 # note the main input file and prefix for all output files
 infile=$DATA/e4.60005.nii.gz
 prefix=out.c22
@@ -42,7 +59,7 @@ ${NT} -copy_image -infile ${prefix}.0.i16.nii.gz \
 ${NT} -copy_image -infile ${prefix}.1.i64.nii.gz   \
                   -prefix ${prefix}.2.0.i16.nii.gz \
                   -convert2dtype NIFTI_TYPE_INT16 -convert_verify
-if cmp ${prefix}.0.i16.nii.gz ${prefix}.2.0.i16.nii.gz
+if nii_cmp ${prefix}.0.i16.nii.gz ${prefix}.2.0.i16.nii.gz
 then
 echo ""
 else
@@ -57,7 +74,7 @@ ${NT} -cbl -infile ${prefix}.0.i16.nii.gz \
 ${NT} -copy_image -infile ${prefix}.1.f32.nii.gz   \
                   -prefix ${prefix}.2.1.i16.nii.gz \
                   -convert2dtype NIFTI_TYPE_INT16 -convert_fail_choice warn
-if cmp ${prefix}.0.i16.nii.gz ${prefix}.2.1.i16.nii.gz
+if nii_cmp ${prefix}.0.i16.nii.gz ${prefix}.2.1.i16.nii.gz
 then
 echo ""
 else