diff --git a/build.gradle b/build.gradle
index 2d7841d..0547b9c 100644
--- a/build.gradle
+++ b/build.gradle
@@ -1,6 +1,6 @@
-
 plugins {
     id 'java-library'
+    id "net.ltgt.errorprone" version "3.1.0"
 }
 
 repositories {
@@ -14,19 +14,30 @@
     }
 }
 
-compileJava {
+tasks.withType(JavaCompile) {
     options.compilerArgs << '-Xlint:unchecked'
     options.compilerArgs << '-Xlint:rawtypes'
     options.release = 21
-}
-
-compileTestJava {
-    options.compilerArgs << '-Xlint:unchecked'
-    options.compilerArgs << '-Xlint:rawtypes'
-    options.release = 21
+    if (!name.toLowerCase().contains("test")) {
+      options.errorprone {
+        check("NullAway", net.ltgt.gradle.errorprone.CheckSeverity.ERROR)
+        option("NullAway:AnnotatedPackages", "com.github.tonivade.vavr.effect")
+        option("NullAway:SuggestSuppressions", "true")
+      }
+    } else {
+      options.errorprone {
+        option("NullAway:AnnotatedPackages", "com.github.tonivade.vavr.effect")
+        option("ExcludedPaths", ".*/src/test/java/.*")
+      }
+    }
 }
 
 dependencies {
+    errorprone 'com.google.errorprone:error_prone_core:2.25.0'
+    errorprone 'com.uber.nullaway:nullaway:0.10.24'
+    
+    compileOnly 'com.google.code.findbugs:jsr305:3.0.2'
+
     api 'io.vavr:vavr:0.10.4'
 
     testImplementation 'org.junit.jupiter:junit-jupiter:5.10.2'
diff --git a/src/main/java/com/github/tonivade/vavr/effect/IO.java b/src/main/java/com/github/tonivade/vavr/effect/IO.java
index 90073c4..2f0df53 100644
--- a/src/main/java/com/github/tonivade/vavr/effect/IO.java
+++ b/src/main/java/com/github/tonivade/vavr/effect/IO.java
@@ -24,6 +24,8 @@
 import java.util.function.Supplier;
 import java.util.function.UnaryOperator;
 
+import javax.annotation.Nullable;
+
 import io.vavr.CheckedConsumer;
 import io.vavr.CheckedFunction0;
 import io.vavr.CheckedRunnable;
@@ -792,7 +794,7 @@
 
   static final class Cancellable implements IOConnection {
 
-    private IO<Unit> cancelToken;
+    private IO<Unit> cancelToken = IO.UNIT;
     private final AtomicReference<StateIO> state = new AtomicReference<>(StateIO.INITIAL);
 
     private Cancellable() { }
@@ -856,13 +858,19 @@
 
 final class CallStack<T> {
 
+  @Nullable
   private StackItem<T> top = new StackItem<>();
 
   void push() {
-    top.push();
+    if (top != null) {
+      top.push();
+    }
   }
 
   void pop() {
+    if (top == null) {
+      return;
+    }
     if (top.count() > 0) {
       top.pop();
     } else {
@@ -871,6 +879,9 @@
   }
 
   void add(PartialFunction<? super Throwable, ? extends IO<? extends T>> mapError) {
+    if (top == null) {
+      return;
+    }
     if (top.count() > 0) {
       top.pop();
       top = new StackItem<>(top);
@@ -904,16 +915,18 @@
   private int count = 0;
   private final Deque<PartialFunction<? super Throwable, ? extends IO<? extends T>>> recover = new ArrayDeque<>();
 
+  @Nullable
   private final StackItem<T> prev;
 
   StackItem() {
     this(null);
   }
 
-  StackItem(StackItem<T> prev) {
+  StackItem(@Nullable StackItem<T> prev) {
     this.prev = prev;
   }
 
+  @Nullable
   StackItem<T> prev() {
     return prev;
   }