diff --git a/src/Processor/Expression/FunctionEvaluator.php b/src/Processor/Expression/FunctionEvaluator.php
index 60c81b6d..c6db5e8f 100644
--- a/src/Processor/Expression/FunctionEvaluator.php
+++ b/src/Processor/Expression/FunctionEvaluator.php
@@ -353,9 +353,13 @@ private static function sqlCount(
     }
 
     /**
-     * @param array<string, Column> $columns
+     * @param FakePdoInterface $conn
+     * @param Scope $scope
+     * @param FunctionExpression $expr
+     * @param QueryResult $result
      *
-     * @return ?numeric
+     * @return float|int|mixed|string|null
+     * @throws ProcessorException
      */
     private static function sqlSum(
         FakePdoInterface $conn,
@@ -368,6 +372,10 @@ private static function sqlSum(
         $sum = 0;
 
         if (!$result->rows) {
+            if ($expr instanceof FunctionExpression) {
+                return self::evaluate($conn, $scope, $expr, [], $result);
+            }
+
             return null;
         }
 
@@ -1575,6 +1583,10 @@ private static function sqlConvertTz(
             throw new \InvalidArgumentException("CONVERT_TZ() requires exactly 3 arguments");
         }
 
+        if ($args[0] instanceof ColumnExpression && empty($row)) {
+            return null;
+        }
+
         /** @var string|null $dtValue */
         $dtValue = Evaluator::evaluate($conn, $scope, $args[0], $row, $result);
         /** @var string|null $fromTzValue */
diff --git a/tests/EndToEndTest.php b/tests/EndToEndTest.php
index bd6118d1..f59be4f0 100644
--- a/tests/EndToEndTest.php
+++ b/tests/EndToEndTest.php
@@ -1516,4 +1516,44 @@ public function leastWithExceptionProvider(): iterable
         yield ['Should fail with single argument' => [1]];
         yield ['Should fail without any arguments' => []];
     }
+
+    public function testNestedFunctions()
+    {
+        $pdo = self::getConnectionToFullDB();
+
+        $query = $pdo->prepare("
+            SELECT 
+                SUM(
+                    TIMESTAMPDIFF(
+                        SECOND,
+                        CONVERT_TZ('2025-12-31 22:59:59', 'Europe/Kyiv', 'Europe/Kyiv'),
+                        CONVERT_TZ('2025-12-31 23:59:59', 'Europe/Kyiv', 'Europe/Kyiv')
+                    )
+                )
+        ");
+        $query->execute();
+
+        $this->assertSame(3600, (int)$query->fetchColumn());
+    }
+
+    public function testNestedFunctionsFromDB()
+    {
+        $pdo = self::getConnectionToFullDB();
+        $count = $pdo->query("SELECT COUNT(*) FROM video_game_characters")->fetchColumn();
+
+        $query = $pdo->prepare("
+            SELECT SUM(
+                    TIMESTAMPDIFF(
+                        SECOND,
+                        CONVERT_TZ(`created_on`, 'Europe/Kyiv', 'Europe/Kyiv'),
+                        CONVERT_TZ(`created_on` + INTERVAL 1 SECOND, 'Europe/Kyiv', 'Europe/Kyiv')
+                    )
+                )
+            FROM `video_game_characters`
+        ");
+
+        $query->execute();
+
+        $this->assertSame((int)$count, (int)$query->fetchColumn());
+    }
 }