diff --git a/later/index.d.ts b/later/index.d.ts
index c5c2449a00..5dd68ec90a 100644
--- a/later/index.d.ts
+++ b/later/index.d.ts
@@ -142,7 +142,7 @@ declare namespace later {
          * For acces to custom time periods created as extension to the later static type
          * and modifiers created on the later modifier static type.
          */
-        [ timeperiodAndModifierName: string ]: number[];
+        [ timeperiodAndModifierName: string ]: number[] | undefined;
     }
 
     interface ParseStatic {
diff --git a/later/tsconfig.json b/later/tsconfig.json
index 78a6cefd34..add4a07667 100644
--- a/later/tsconfig.json
+++ b/later/tsconfig.json
@@ -7,7 +7,7 @@
         ],
         "noImplicitAny": true,
         "noImplicitThis": true,
-        "strictNullChecks": false,
+        "strictNullChecks": true,
         "baseUrl": "../",
         "typeRoots": [
             "../"