diff --git a/test/spec/aggregate.test.ts b/test/spec/aggregate.test.ts
index a62bdb4..8266992 100644
--- a/test/spec/aggregate.test.ts
+++ b/test/spec/aggregate.test.ts
@@ -13,7 +13,7 @@ interface IAggregateSigsTestCase {
 }
 
 describeForAllImplementations((bls) => {
-  describeDirectorySpecTest<IAggregateSigsTestCase, string>(
+  describeDirectorySpecTest<IAggregateSigsTestCase, string | null>(
     "bls/aggregate/small",
     path.join(SPEC_TESTS_DIR, "tests/general/phase0/bls/aggregate/small"),
     (testCase) => {
diff --git a/test/spec/sign.test.ts b/test/spec/sign.test.ts
index 5a5f835..3d17d60 100644
--- a/test/spec/sign.test.ts
+++ b/test/spec/sign.test.ts
@@ -16,7 +16,7 @@ interface ISignMessageTestCase {
 }
 
 describeForAllImplementations((bls) => {
-  describeDirectorySpecTest<ISignMessageTestCase, string>(
+  describeDirectorySpecTest<ISignMessageTestCase, string | null>(
     "bls/sign/small",
     path.join(SPEC_TESTS_DIR, "tests/general/phase0/bls/sign/small"),
     (testCase) => {
diff --git a/tsconfig.json b/tsconfig.json
index 5b70769..843240c 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -8,7 +8,8 @@
     "typeRoots": ["./node_modules/@types"],
     "declaration": true,
     "strict": true,
-    "strictNullChecks": false,
+    "strictNullChecks": true,
+    "strictFunctionTypes": true,
     "esModuleInterop": true
   }
 }