diff --git a/src/asserts/callback-assert.js b/src/asserts/callback-assert.js
index 2026ff8..88ff8fb 100644
--- a/src/asserts/callback-assert.js
+++ b/src/asserts/callback-assert.js
@@ -11,7 +11,7 @@ const { Violation } = require('validator.js');
  * Constants.
  */
 
-const expression = /^[a-zA-Z\d]+$/;
+const expression = /^[a-zA-Z\d_]+$/;
 
 /**
  * Export `CallbackAssert`.
diff --git a/test/asserts/callback-assert.test.js b/test/asserts/callback-assert.test.js
index 3005aa0..80d8cde 100644
--- a/test/asserts/callback-assert.test.js
+++ b/test/asserts/callback-assert.test.js
@@ -91,6 +91,14 @@ describe('CallbackAssert', () => {
     }
   });
 
+  it('should expose `assert` equal to `Custom_Class1`', () => {
+    try {
+      Assert.callback(value => value === 'foobiz', 'Custom_Class1').validate('foobar');
+    } catch (e) {
+      expect(e.show().assert).toEqual('Custom_Class1');
+    }
+  });
+
   it('should not throw an error if the callback function returns `true`', () => {
     Assert.callback(value => value === 'foobar', 'CustomClass').validate('foobar');
   });