diff --git a/internal/config/config.go b/internal/config/config.go
index d091bd4..699003b 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -86,8 +86,8 @@ func load(lookup func(string) (string, bool)) (Config, error) {
 		OIDCClientSecret:  r.secretFile(EnvOIDCClientSecretFile),
 		OIDCAllowedGroups: r.list(EnvOIDCAllowedGroups),
 
-		SessionIdle: r.optionalDuration(EnvSessionIdle, DefaultSessionIdle),
-		SessionMax:  r.optionalDuration(EnvSessionMax, DefaultSessionMax),
+		SessionIdle: r.sessionDuration(EnvSessionIdle, DefaultSessionIdle),
+		SessionMax:  r.sessionDuration(EnvSessionMax, DefaultSessionMax),
 	}
 	if cfg.SessionIdle > 0 && cfg.SessionMax > 0 && cfg.SessionMax < cfg.SessionIdle {
 		r.fail(EnvSessionMax, "must be at least %s (%s)", EnvSessionIdle, cfg.SessionIdle)
@@ -277,6 +277,18 @@ func (r *reader) optionalDuration(name string, def time.Duration) time.Duration
 	return r.parseDuration(name, v)
 }
 
+// sessionDuration is optionalDuration restricted to whole milliseconds:
+// session expiry is stored as a store.Timestamp, which keeps millisecond
+// precision, so a finer duration could not be honoured.
+func (r *reader) sessionDuration(name string, def time.Duration) time.Duration {
+	d := r.optionalDuration(name, def)
+	if d%time.Millisecond != 0 {
+		r.fail(name, "duration must be a whole number of milliseconds, got %s", d)
+		return 0
+	}
+	return d
+}
+
 func (r *reader) parseDuration(name, v string) time.Duration {
 	d, err := time.ParseDuration(v)
 	if err != nil {
diff --git a/internal/config/config_test.go b/internal/config/config_test.go
index bd8feae..4dafec1 100644
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -269,6 +269,20 @@ func TestLoad(t *testing.T) {
 			env:     with(map[string]string{EnvSessionMax: "-1h"}),
 			badVars: []string{EnvSessionMax},
 		},
+		{
+			name: "whole-millisecond session lifetimes",
+			env:  with(map[string]string{EnvSessionIdle: "1ms", EnvSessionMax: "2ms"}),
+		},
+		{
+			name:    "sub-millisecond session idle",
+			env:     with(map[string]string{EnvSessionIdle: "1500us", EnvSessionMax: "2ms"}),
+			badVars: []string{EnvSessionIdle},
+		},
+		{
+			name:    "sub-millisecond session max",
+			env:     with(map[string]string{EnvSessionIdle: "1ms", EnvSessionMax: "1ms500us"}),
+			badVars: []string{EnvSessionMax},
+		},
 		{
 			name:    "session max below idle",
 			env:     with(map[string]string{EnvSessionIdle: "48h", EnvSessionMax: "24h"}),