diff --git a/Sources/Core/Domain/FSRSScheduler.swift b/Sources/Core/Domain/FSRSScheduler.swift
index b88488c..9174de5 100644
--- a/Sources/Core/Domain/FSRSScheduler.swift
+++ b/Sources/Core/Domain/FSRSScheduler.swift
@@ -73,10 +73,20 @@ struct FSRS {
     private let intervalModifier: Double
 
     init(_ settings: FSRSSettings = FSRSSettings()) {
-        s = settings
-        decay = -settings.w[20]
-        factor = Self.roundTo(exp((1.0 / (-settings.w[20])) * log(0.9)) - 1, 8)
-        intervalModifier = Self.roundTo((pow(settings.requestRetention, 1.0 / (-settings.w[20])) - 1) / factor, 8)
+        var resolved = settings
+        // FSRS-5-Settings haben 19 Gewichte → auf FSRS-6 (21) migrieren,
+        // sonst crasht `w[20]`. Bei jeder anderen unerwarteten Länge sicher
+        // auf die Defaults zurückfallen.
+        if resolved.w.count == 19 {
+            resolved.w.append(contentsOf: [0.0658, 0.1542])
+        }
+        if resolved.w.count != 21 {
+            resolved.w = Self.defaultW
+        }
+        s = resolved
+        decay = -resolved.w[20]
+        factor = Self.roundTo(exp((1.0 / decay) * log(0.9)) - 1, 8)
+        intervalModifier = Self.roundTo((pow(resolved.requestRetention, 1.0 / decay) - 1) / factor, 8)
     }
 
     // MARK: - Public API
diff --git a/Tests/UnitTests/FSRSSchedulerTests.swift b/Tests/UnitTests/FSRSSchedulerTests.swift
index 9905330..a43bbd7 100644
--- a/Tests/UnitTests/FSRSSchedulerTests.swift
+++ b/Tests/UnitTests/FSRSSchedulerTests.swift
@@ -84,6 +84,16 @@ struct FSRSSchedulerTests {
             }
         }
     }
+
+    @Test("FSRS-5-Settings (19 Gewichte) crashen nicht, werden migriert")
+    func fsrs5WeightsMigrateInsteadOfCrash() {
+        var settings = FSRSSettings()
+        settings.w = Array(FSRS.defaultW.prefix(19)) // FSRS-5-Format
+        let fsrs = FSRS(settings)
+        let next = fsrs.grade(card: .empty(now: Date()), rating: .good, now: Date())
+        #expect(next.reps == 1)
+        #expect(next.state != .new)
+    }
 }
 
 // swiftlint:enable identifier_name