diff --git a/api/validators/TotpValidator.php b/api/validators/TotpValidator.php
index 9b5f277..e436d0c 100644
--- a/api/validators/TotpValidator.php
+++ b/api/validators/TotpValidator.php
@@ -57,15 +57,16 @@ class TotpValidator extends Validator {
     }
 
     private function getTimestamp(): ?int {
-        if ($this->timestamp === null) {
+        $timestamp = $this->timestamp;
+        if (is_callable($timestamp)) {
+            $timestamp = call_user_func($this->timestamp);
+        }
+
+        if ($timestamp === null) {
             return null;
         }
 
-        if (is_callable($this->timestamp)) {
-            return (int)call_user_func($this->timestamp);
-        }
-
-        return (int)$this->timestamp;
+        return (int)$timestamp;
     }
 
 }
diff --git a/tests/codeception/api/unit/validators/TotpValidatorTest.php b/tests/codeception/api/unit/validators/TotpValidatorTest.php
index 9385356..dfc0bd3 100644
--- a/tests/codeception/api/unit/validators/TotpValidatorTest.php
+++ b/tests/codeception/api/unit/validators/TotpValidatorTest.php
@@ -39,6 +39,13 @@ class TotpValidatorTest extends TestCase {
         $result = $this->callProtected($validator, 'validateValue', $controlTotp->at($at));
         $this->assertNull($result);
 
+        $at = function() {
+            return null;
+        };
+        $validator->timestamp = $at;
+        $result = $this->callProtected($validator, 'validateValue', $controlTotp->now());
+        $this->assertNull($result);
+
         $at = function() {
             return time() - 700;
         };