diff --git a/src/Mutator/Extensions/MBString.php b/src/Mutator/Extensions/MBString.php
index 9b789792c..a7b51e4f5 100644
--- a/src/Mutator/Extensions/MBString.php
+++ b/src/Mutator/Extensions/MBString.php
@@ -163,6 +163,8 @@ private function getConvertCaseModeValue(Node\Expr\FuncCall $node): ?int
             if (isset(self::MB_CASES[$modeName])) {
                 return self::MB_CASES[$modeName];
             }
+        } elseif ($mode instanceof Node\Scalar\LNumber) {
+            return $mode->value;
         }
 
         return null;
diff --git a/tests/Mutator/Extensions/MBStringTest.php b/tests/Mutator/Extensions/MBStringTest.php
index 5bd128fce..14ed4e064 100644
--- a/tests/Mutator/Extensions/MBStringTest.php
+++ b/tests/Mutator/Extensions/MBStringTest.php
@@ -414,11 +414,16 @@ private function provideMutationCasesForConvertCase(): Generator
             "<?php\n\nstrtoupper('test');",
         ];
 
-        yield 'It converts mb_convert_case with MB_CASE_LOWER mode to strtolower' => [
+        yield 'It converts mb_convert_case with constant similar MB_CASE_LOWER mode to strtolower' => [
             "<?php mb_convert_case('test', E_ERROR);",
             "<?php\n\nstrtolower('test');",
         ];
 
+        yield 'It converts mb_convert_case with numeric MB_CASE_LOWER (1) mode to strtolower' => [
+            "<?php mb_convert_case('test', 1);",
+            "<?php\n\nstrtolower('test');",
+        ];
+
         yield 'It converts mb_convert_case with MB_CASE_LOWER_SIMPLE mode to strtolower' => [
             "<?php mb_convert_case('test', MB_CASE_LOWER);",
             "<?php\n\nstrtolower('test');",