diff --git a/test/harness_selftest.sh b/test/harness_selftest.sh
index 1fa6dfcb..a4f38ed8 100755
--- a/test/harness_selftest.sh
+++ b/test/harness_selftest.sh
@@ -305,11 +305,59 @@ rm -f "$_fx"
 #
 # This check is what keeps the property true. Without it the order decays the
 # first time somebody appends by hand, and the reduction quietly goes away.
-_sorted_expected="$(listed_suites | sort)"
+#
+# LC_ALL=C, and the collation is part of the property rather than a detail (#552).
+# "Sorted" is not machine-independent: C compares byte by byte so `_` (0x5F)
+# precedes `e`, while en_US.UTF-8 ignores punctuation at the first level and
+# compares `sortstatus` against `sortedprojection`. The array holds sort_status
+# then sorted_projection, so it is sorted in one and unsorted in the other, and
+# this check pinned neither. On a clean main it FAILED under en_US.UTF-8 and
+# passed here only because the container defaults to C.UTF-8, which collates
+# like C.
+#
+# The ambiguity is worse than the false red. Sorted order is what gives two
+# agents' new suites different insertion points; if two contributors disagree
+# about what sorted means they insert in different places, and the property stops
+# delivering the merges it exists for.
+_sorted_expected="$(listed_suites | LC_ALL=C sort)"
 _sorted_actual="$(listed_suites)"
-check "the suite list is sorted, so two new suites land in different places" \
+check "the suite list is sorted in C order, so two new suites land in different places" \
 	"$([ "$_sorted_actual" = "$_sorted_expected" ] && echo sorted || echo "not sorted")" "sorted"
 
+# The pair that decides it, asserted directly so a future edit that "fixes" the
+# order to UTF-8 collation fails here with the reason rather than only failing
+# the comparison above.
+check "premise: C collation puts sort_status before sorted_projection" \
+	"$(printf 'sorted_projection\nsort_status\n' | LC_ALL=C sort | head -1)" "sort_status"
+
+# ---- and comm's two inputs must be sorted the SAME way (#552 follow-up) -----
+#
+# `comm` requires both inputs sorted in one collation and does not check. Fed
+# inconsistently-sorted input it does not error; it returns the wrong lines.
+#
+# test/rebuild.sh:130 does `comm -23` over two `sort -u` outputs, neither pinned.
+# They agree today because they share a locale. The plausible next edit is
+# somebody pinning ONE of them because this PR taught them to, and the result is
+# a symbol check that silently reports the wrong unresolved symbols -- either a
+# false red, or the worse direction, a real unresolved symbol not reported.
+#
+# Asserted over source text, which is the weaker kind, because reproducing it
+# needs two locales and a built .so. Premised on the comm still existing, or the
+# grep approves a file that no longer has one.
+_cm_files="$(grep -ln 'comm -' "$(dirname "${BASH_SOURCE[0]}")"/*.sh 2>/dev/null)"
+check "premise: some suite still uses comm, or the check below is vacuous" \
+	"$([ -n "$_cm_files" ] && echo yes || echo no)" "yes"
+
+_cm_unpinned=""
+for _f in $_cm_files; do
+	# every `| sort` in a file that uses comm must carry LC_ALL=C
+	if grep -qE '\|[[:space:]]*sort' "$_f" && grep -E '\|[[:space:]]*sort' "$_f" | grep -qv 'LC_ALL=C'; then
+		_cm_unpinned="$_cm_unpinned $(basename "$_f")"
+	fi
+done
+check "a file that uses comm pins the collation of every sort feeding it" \
+	"$(printf '%s' "$_cm_unpinned" | sed 's/^ //')" ""
+
 # A case over the cached list rather than `listed_suites | grep -qx`. The pipe
 # was the defect: grep -q returns on its match, printf takes EPIPE, and pipefail
 # turns that into a failed pipeline for a suite that IS registered. See the note
diff --git a/test/rebuild.sh b/test/rebuild.sh
index e1f4afd9..60499a1f 100755
--- a/test/rebuild.sh
+++ b/test/rebuild.sh
@@ -116,18 +116,21 @@ if command -v nm >/dev/null 2>&1; then
 	IGNORE='^(_ITM_|__gmon_start__$|__cxa_finalize$)'
 
 	undef="$(nm -D --undefined-only "$SO" 2>/dev/null | awk '{print $NF}' |
-		strip_ver | grep -Ev "$IGNORE" | sort -u)"
+		strip_ver | grep -Ev "$IGNORE" | LC_ALL=C sort -u)"
 	# The .so is dlopen'd into the running postgres, so its symbols resolve against
 	# the server binary, everything the server itself links (libm, libssl, ...), and
 	# the .so's own dependencies. All three belong in the reference set.
 	defined="$(nm -D --defined-only "$BINDIR/postgres" 2>/dev/null | awk '{print $NF}')"
 	for lib in $(ldd "$SO" "$BINDIR/postgres" 2>/dev/null |
-			awk '/=>/ {print $3}' | grep -v '^$' | sort -u); do
+			awk '/=>/ {print $3}' | grep -v '^$' | LC_ALL=C sort -u); do
 		defined="$defined
 $(nm -D --defined-only "$lib" 2>/dev/null | awk '{print $NF}')"
 	done
-	defined="$(echo "$defined" | strip_ver | sort -u)"
-	missing="$(comm -23 <(echo "$undef") <(echo "$defined"))"
+	defined="$(echo "$defined" | strip_ver | LC_ALL=C sort -u)"
+	# comm requires ONE collation across both inputs and does not check. Both
+	# sorts above are pinned to C, so comm is pinned to C too -- inputs sorted one
+	# way and compared another is the same defect with an extra step (#552).
+	missing="$(LC_ALL=C comm -23 <(echo "$undef") <(echo "$defined"))"
 	if [ -n "$missing" ]; then
 		echo "rebuild: UNRESOLVED SYMBOLS against $PGVER:" >&2
 		echo "$missing" | head -20 >&2