diff --git a/scripts/governance-check.sh b/scripts/governance-check.sh
index b0a9dfb..effae29 100755
--- a/scripts/governance-check.sh
+++ b/scripts/governance-check.sh
@@ -41,6 +41,16 @@ if ! git rev-parse --verify --quiet "${BASE_REF}^{commit}" >/dev/null; then
   exit 1
 fi
 
+# Diff the change set from the merge base, not from the base tip. A two-dot
+# diff against a moving base reports everything the base gained after the
+# branch point as if this change reverted it, so a branch behind main that
+# touches C# would fail G13 whenever main had merged Terraform in the meantime.
+# The merge queue no longer requires branches to be current, so this matters.
+DIFF_BASE="$(git merge-base "${BASE_REF}" "${HEAD_REF}")" || {
+  bad "G3: no merge base between '${BASE_REF}' and '${HEAD_REF}'."
+  exit 1
+}
+
 log "G1: restore"
 "$DOTNET" restore "$SOLUTION"
 ok "G1: restore"
@@ -52,16 +62,16 @@ log "G2: architecture boundary tests (dependency direction)"
   --nologo
 ok "G2: ArchitectureTests"
 
-log "G3: changed-file formatting (${BASE_REF}..${HEAD_REF})"
+log "G3: changed-file formatting (${BASE_REF}...${HEAD_REF}, merge base ${DIFF_BASE:0:7})"
 changed_cs=()
 while IFS= read -r f; do
   changed_cs+=("$f")
 done < <(
-  git diff --name-only --diff-filter=ACMR "${BASE_REF}" "${HEAD_REF}" -- '*.cs'
+  git diff --name-only --diff-filter=ACMR "${DIFF_BASE}" "${HEAD_REF}" -- '*.cs'
 )
 
 if (( ${#changed_cs[@]} == 0 )); then
-  printf '\033[33mSKIP\033[0m  G3: no changed C# files between %s..%s\n' "${BASE_REF}" "${HEAD_REF}"
+  printf '\033[33mSKIP\033[0m  G3: no changed C# files between %s...%s\n' "${BASE_REF}" "${HEAD_REF}"
 else
   printf '  checking %d changed C# file(s)\n' "${#changed_cs[@]}"
   "$DOTNET" format "$SOLUTION" \
@@ -89,9 +99,9 @@ python3 scripts/test_require_commit_checks.py
 python3 scripts/test_check_app_terraform_isolation.py
 ok "Release promotion scripts"
 
-log "G13: application and Terraform isolation (${BASE_REF}..${HEAD_REF})"
+log "G13: application and Terraform isolation (${BASE_REF}...${HEAD_REF}, merge base ${DIFF_BASE:0:7})"
 python3 scripts/check_app_terraform_isolation.py < <(
-  git diff --name-only --diff-filter=ACMR "${BASE_REF}" "${HEAD_REF}"
+  git diff --name-only --diff-filter=ACMR "${DIFF_BASE}" "${HEAD_REF}"
 )
 ok "G13: application and Terraform isolation"