diff --git a/lib/prototypes/abstract.js b/lib/prototypes/abstract.js
index 2e8e72a..cf485f4 100644
--- a/lib/prototypes/abstract.js
+++ b/lib/prototypes/abstract.js
@@ -27,14 +27,14 @@ class AbstractType {
   }
 
   check(value, path) {
-    return this.commonCheck(value, path, false);
+    return this.#commonCheck(value, path, false);
   }
 
   partialCheck(value, path) {
-    return this.commonCheck(value, path, true);
+    return this.#commonCheck(value, path, true);
   }
 
-  commonCheck(value, path, isPartial) {
+  #commonCheck(value, path, isPartial) {
     const result = new ValidationResult(path);
     const isEmpty = value === null || value === undefined;
     const isRequiredAndMissing = !isPartial
diff --git a/lib/struct.js b/lib/struct.js
index 0bfada5..9742e17 100644
--- a/lib/struct.js
+++ b/lib/struct.js
@@ -20,14 +20,14 @@ class Struct {
   }
 
   check(source, path = '') {
-    return this.commonCheck(source, path, false);
+    return this.#commonCheck(source, path, false);
   }
 
   partialCheck(source, path = '') {
-    return this.commonCheck(source, path, true);
+    return this.#commonCheck(source, path, true);
   }
 
-  commonCheck(source, path, isPartial) {
+  #commonCheck(source, path, isPartial) {
     const result = new ValidationResult(path || this.name);
     const keys = Object.keys(source);
     const fields = !isPartial ? Object.keys(this) : [];