diff --git a/README.md b/README.md
index d1cbdf4..4d188b5 100644
--- a/README.md
+++ b/README.md
@@ -1449,11 +1449,12 @@ whole service, its `asn`, for an AS number, its `name`, for a named netblock, or
 the `start` of a window in which it has requests or bytes; a list's `url`,
 `fetched` or `lines`, which is `[]` for an empty list; a verdict's `zone`,
 `client`, `listed`, which is `false` for a client the zone does not list, or
-`fetched`. So does a ban whose `cause` is not `limit`, `attack`, `admin` or
-`crowdsec`, alerts waiting for a destination that is not `webhook`, `slack` or
-`ntfy`, an anomaly counter whose `scope` is not `client`, `net`, `asn`, `total`
-or `watch`, and a copy of a list with a line that would make its fetch fail. An
-answer's `asn` or `as_name` left out reads as empty.
+`fetched`. So does a ban whose `expires` is not after its `start` or whose
+`cause` is not `limit`, `attack`, `admin` or `crowdsec`, alerts waiting for a
+destination that is not `webhook`, `slack` or `ntfy`, an anomaly counter whose
+`scope` is not `client`, `net`, `asn`, `total` or `watch`, and a copy of a list
+with a line that would make its fetch fail. An answer's `asn` or `as_name` left
+out reads as empty.
 
 While it runs, `smallwebwaf` watches `SWWAF_STATE_DIR` and takes in your edit of
 a state file as soon as you save it: what the file then holds replaces what
@@ -1463,14 +1464,15 @@ writes a file it takes in any edit made since, so your edit is not overwritten;
 a change `smallwebwaf` made after you opened the file, such as a new ban, is
 lost when you save over it. An edit that would stop the start, because it does
 not parse, has another `version`, leaves out a field an entry needs, gives a ban
-another `cause`, names another destination, gives an anomaly counter another
-`scope` or gives a list's copy a line that would make its fetch fail, does not
-stop the running `smallwebwaf`: it keeps what it holds, and at the file's next
-write renames your file to `<name>.bad`, such as `bans.json.bad`, writes the
-file again from memory, logs the file and where the error is, and raises a
-`file_error` alert for it. It waits for that write because an editor's file can
-be read before the editor has finished writing it. Mend the `.bad` file and move
-it back. A file you remove is written again at its next write.
+an `expires` not after its `start` or another `cause`, names another
+destination, gives an anomaly counter another `scope` or gives a list's copy a
+line that would make its fetch fail, does not stop the running `smallwebwaf`: it
+keeps what it holds, and at the file's next write renames your file to
+`<name>.bad`, such as `bans.json.bad`, writes the file again from memory, logs
+the file and where the error is, and raises a `file_error` alert for it. It
+waits for that write because an editor's file can be read before the editor has
+finished writing it. Mend the `.bad` file and move it back. A file you remove is
+written again at its next write.
 
 To ban a netblock, add an entry to `bans.json` with its `netblock`, its `start`
 and its `expires`, `null` for a ban that never ends; its `reason` and its
diff --git a/internal/state/state.go b/internal/state/state.go
index b662075..a6321c4 100644
--- a/internal/state/state.go
+++ b/internal/state/state.go
@@ -61,6 +61,7 @@ var (
 	// errMissing is for an entry without a field it needs.
 	errMissing     = errors.New("has no")
 	errCause       = errors.New("is not limit, attack, admin or crowdsec")
+	errExpires     = errors.New("is not after its start")
 	errDestination = errors.New("is not webhook, slack or ntfy")
 	errScope       = errors.New("is not client, net, asn, total or watch")
 	errWaitingList = errors.New(`waiting is a list, but now lists the alerts by ` +
@@ -646,8 +647,10 @@ func (e BanEntry) ban() bans.Ban {
 // client, a start, from which the length of the netblock's next ban is
 // worked out, or an expires, which would make it permanent. A permanent
 // ban's expires is null, which Bans cannot tell from a missing one, so
-// each expires is read again as written. A cause other than limit,
-// attack, admin or crowdsec, most likely misspelt, is refused too.
+// each expires is read again as written. An expires not after the start
+// is refused too, since the netblock's next ban for a broken limit, three
+// times as long as this one, would then refuse nothing; and so is a cause
+// other than limit, attack, admin or crowdsec, most likely misspelt.
 func (f *bansFile) check(data []byte) error {
 	var written struct {
 		Bans []struct {
@@ -668,6 +671,8 @@ func (f *bansFile) check(data []byte) error {
 			return missing(i, "start")
 		case written.Bans[i].Expires == nil:
 			return missing(i, "expires")
+		case entry.Expires != nil && !entry.Expires.After(entry.Start):
+			return fmt.Errorf("entry %d's expires %w", i+1, errExpires)
 		case entry.Cause != "" && entry.Cause != bans.CauseLimit &&
 			entry.Cause != bans.CauseAttack && entry.Cause != bans.CauseAdmin &&
 			entry.Cause != bans.CauseCrowdSec:
diff --git a/internal/state/state_test.go b/internal/state/state_test.go
index 0a95f2e..0b98d94 100644
--- a/internal/state/state_test.go
+++ b/internal/state/state_test.go
@@ -865,6 +865,21 @@ func TestBanWithAnotherCauseStopsTheStart(t *testing.T) {
 		`: entry 4's cause "atack" is not limit, attack, admin or crowdsec`)
 }
 
+func TestBanThatDoesNotEndAfterItsStartStopsTheStart(t *testing.T) {
+	t.Parallel()
+
+	// The first ban ends a second after its start; the second ends as it
+	// starts, or an hour before.
+	for _, expires := range []string{`"2026-10-06T00:00:00Z"`, `"2026-10-05T23:00:00Z"`} {
+		wantRefused(t, bansJSON, `{"version": 1, "bans": [`+
+			`{"netblock": "203.0.113.9/32", "start": "2026-10-06T00:00:00Z", `+
+			`"expires": "2026-10-06T00:00:01Z"}, `+
+			`{"netblock": "203.0.113.10/32", "start": "2026-10-06T00:00:00Z", `+
+			`"expires": `+expires+`}]}`,
+			`: entry 2's expires is not after its start`)
+	}
+}
+
 func TestUnknownVersionStopsTheStart(t *testing.T) {
 	t.Parallel()