diff --git a/src/env/env.manager.ts b/src/env/env.manager.ts
index ecf632d..901b931 100644
--- a/src/env/env.manager.ts
+++ b/src/env/env.manager.ts
@@ -7,7 +7,7 @@ interface Options<NodeEnv, Env> {
 }
 
 export class EnvManager<NodeEnv extends string, Config extends BaseConfig = BaseConfig> extends Manager<Config> {
-  private nodeEnv: string;
+  private nodeEnv: NodeEnv;
 
   constructor({
     getEnv = () => process.env as Config,
@@ -18,7 +18,7 @@ export class EnvManager<NodeEnv extends string, Config extends BaseConfig = Base
     this.nodeEnv = getNodeEnv();
   }
 
-  private getEnvValue<Key extends keyof Config>(key: Key) {
+  private getEnvValue<Key extends keyof Config>(key: Key): Config[Key] | undefined {
     const value = this.source[key];
 
     if (value === null || value === undefined || value.trim() === '') {
@@ -28,8 +28,8 @@ export class EnvManager<NodeEnv extends string, Config extends BaseConfig = Base
     return value;
   }
 
-  private getKeyOrThrow<Key extends keyof Config>(envRecord: EnvRecord<NodeEnv, Key>) {
-    const key: Key | undefined = envRecord[this.nodeEnv as NodeEnv] ?? envRecord.rest;
+  private getKeyOrThrow<Key extends keyof Config>(envRecord: EnvRecord<NodeEnv, Key>): Key {
+    const key = envRecord[this.nodeEnv] ?? envRecord.rest;
 
     if (!key) {
       throw new Error(`key is not found empty in nodeEnv: ${this.nodeEnv}`);
@@ -38,38 +38,42 @@ export class EnvManager<NodeEnv extends string, Config extends BaseConfig = Base
     return key;
   }
 
-  public pick<Key extends keyof Config>(key: Key) {
-    return new EnvPicker(this.getEnvValue(key), this.nodeEnv) as EnvPicker<NodeEnv, Config[Key] | undefined>;
+  public pick<Key extends keyof Config>(key: Key): EnvPicker<NodeEnv, Config[Key] | undefined> {
+    return new EnvPicker(this.getEnvValue(key), this.nodeEnv);
   }
 
-  public pickOrThrow<Key extends keyof Config>(key: Key) {
+  public pickOrThrow<Key extends keyof Config>(key: Key): EnvPicker<NodeEnv, NonNullable<Config[Key]>> {
     const value = this.source[key];
 
     if (value === null || value === undefined || value.trim() === '') {
       throw new Error(`key: ${key.toString()} is empty`);
     }
 
-    return this.pick(key) as EnvPicker<NodeEnv, NonNullable<Config[Key]>>;
+    return new EnvPicker(value, this.nodeEnv);
   }
 
-  public pickFor<Key extends keyof Config>(envRecord: EnvRecord<NodeEnv, Key>) {
-    const key: Key | undefined = envRecord[this.nodeEnv as NodeEnv] ?? envRecord.rest;
+  public pickFor<Key extends keyof Config>(
+    envRecord: EnvRecord<NodeEnv, Key>,
+  ): EnvPicker<NodeEnv, Config[Key] | undefined> {
+    const key = envRecord[this.nodeEnv] ?? envRecord.rest;
 
     if (!key) {
-      return new EnvPicker(undefined, this.nodeEnv) as EnvPicker<NodeEnv, undefined>;
+      return new EnvPicker(undefined, this.nodeEnv);
     }
 
-    return this.pick(key) as EnvPicker<NodeEnv, Config[Key] | undefined>;
+    return this.pick(key);
   }
 
-  public pickForOrThrow<Key extends keyof Config>(envRecord: EnvRecord<NodeEnv, Key>) {
+  public pickForOrThrow<Key extends keyof Config>(
+    envRecord: EnvRecord<NodeEnv, Key>,
+  ): EnvPicker<NodeEnv, NonNullable<Config[Key]>> {
     const key = this.getKeyOrThrow(envRecord);
 
     return this.pickOrThrow(key);
   }
 
-  public getFor<Key extends keyof Config>(envRecord: EnvRecord<NodeEnv, Key>) {
-    const key: Key | undefined = envRecord[this.nodeEnv as NodeEnv] ?? envRecord.rest;
+  public getFor<Key extends keyof Config>(envRecord: EnvRecord<NodeEnv, Key>): Config[Key] | undefined {
+    const key = envRecord[this.nodeEnv] ?? envRecord.rest;
 
     if (!key) {
       return;
@@ -78,8 +82,8 @@ export class EnvManager<NodeEnv extends string, Config extends BaseConfig = Base
     return this.get(key);
   }
 
-  public getForOrThrow<Key extends keyof Config>(envRecord: EnvRecord<NodeEnv, Key>) {
-    const key: Key | undefined = this.getKeyOrThrow(envRecord);
+  public getForOrThrow<Key extends keyof Config>(envRecord: EnvRecord<NodeEnv, Key>): Config[Key] {
+    const key = this.getKeyOrThrow(envRecord);
 
     return this.getOrThrow(key);
   }
diff --git a/src/env/env.picker.ts b/src/env/env.picker.ts
index 6f9e011..809e00e 100644
--- a/src/env/env.picker.ts
+++ b/src/env/env.picker.ts
@@ -25,13 +25,15 @@ export const wrapInEnvPickers = <NodeEnv extends string, Config extends BaseConf
 export class EnvPicker<NodeEnv extends string, State> {
   constructor(private state: State, private nodeEnv: NodeEnv) {}
 
-  public default(newState: State) {
+  public default(newState: State): EnvPicker<NodeEnv, NonNullable<State>> {
     this.state ??= newState;
 
-    return this as unknown as EnvPicker<NodeEnv, NonNullable<State>>;
+    return this as EnvPicker<NodeEnv, NonNullable<State>>;
   }
 
-  public defaultFor<E extends EnvRecord<NodeEnv, State>>(envRecord: E) {
+  public defaultFor<E extends EnvRecord<NodeEnv, State>>(
+    envRecord: E,
+  ): E extends { rest: State } ? EnvPicker<NodeEnv, NonNullable<State>> : EnvPicker<NodeEnv, State> {
     this.state ??= (envRecord[this.nodeEnv] ?? envRecord.rest) as State;
 
     return this as unknown as E extends { rest: State }
@@ -39,13 +41,15 @@ export class EnvPicker<NodeEnv extends string, State> {
       : EnvPicker<NodeEnv, State>;
   }
 
-  public map<Result>(mapper: (state: State) => Result) {
+  public map<Result>(mapper: (state: State) => Result): EnvPicker<NodeEnv, Result> {
     this.state = mapper(this.state) as unknown as State;
 
     return this as unknown as EnvPicker<NodeEnv, Result>;
   }
 
-  public mapIfExists<Result>(mapper: (state: NonNullable<State>) => Result) {
+  public mapIfExists<Result>(
+    mapper: (state: NonNullable<State>) => Result,
+  ): EnvPicker<NodeEnv, MoveOptional<State, Result>> {
     if (this.state !== null && this.state !== undefined) {
       this.state = mapper(this.state as NonNullable<State>) as unknown as State;
     }
@@ -53,7 +57,7 @@ export class EnvPicker<NodeEnv extends string, State> {
     return this as unknown as EnvPicker<NodeEnv, MoveOptional<State, Result>>;
   }
 
-  public value() {
+  public value(): State {
     return this.state;
   }
 }
diff --git a/src/utils/manager.utils.ts b/src/utils/manager.utils.ts
index abb4550..da4e98e 100644
--- a/src/utils/manager.utils.ts
+++ b/src/utils/manager.utils.ts
@@ -9,11 +9,11 @@ export class Manager<Source extends Record<string, unknown>> {
     this.source = getSource();
   }
 
-  public get<Key extends keyof Source>(key: Key) {
+  public get<Key extends keyof Source>(key: Key): Source[Key] | undefined {
     return this.source[key];
   }
 
-  public getOrThrow<Key extends keyof Source>(key: Key) {
+  public getOrThrow<Key extends keyof Source>(key: Key): Source[Key] {
     const value = this.source[key];
 
     if (value === null || value === undefined) {