diff --git a/ext/mbstring/JitMbStrlen.php b/ext/mbstring/JitMbStrlen.php
index 606ff3ab1fb..5a487d9c611 100644
--- a/ext/mbstring/JitMbStrlen.php
+++ b/ext/mbstring/JitMbStrlen.php
@@ -7,6 +7,7 @@
 use PHPCompiler\ext\standard\VmString;
 use PHPCompiler\JIT\Builtin\StringUtf8Strlen;
 use PHPCompiler\JIT\Context;
+use PHPCompiler\JIT\JitStringBuiltinArg;
 use PHPCompiler\JIT\Variable as JITVariable;
 use PHPLLVM\Value;
 
@@ -15,26 +16,27 @@
  */
 final class JitMbStrlen
 {
-    public static function utf8Length(Context $context, JITVariable $arg): Value
+    public static function utf8LengthFromPtr(Context $context, Value $strPtr): Value
     {
-        if (JITVariable::TYPE_STRING !== $arg->type) {
-            throw new \LogicException('mb_strlen() only supports strings in this compiler build');
-        }
+        StringUtf8Strlen::ensureLinked($context);
+
+        return $context->builder->call(
+            $context->lookupFunction('__compiler_utf8_strlen'),
+            $strPtr
+        );
+    }
 
-        $literal = $arg->compileTimeString ?? null;
-        if (null !== $literal) {
+    public static function utf8Length(Context $context, JITVariable $arg): Value
+    {
+        if (JITVariable::TYPE_STRING === $arg->type && null !== ($arg->compileTimeString ?? null)) {
             return $context->constantFromInteger(
-                VmString::utf8CharLength($literal),
+                VmString::utf8CharLength($arg->compileTimeString),
                 'int64'
             );
         }
 
-        StringUtf8Strlen::ensureLinked($context);
-        $strPtr = $context->helper->loadValue($arg);
+        $str = JitStringBuiltinArg::lower($context, $arg, 'mb_strlen', 0, 'string');
 
-        return $context->builder->call(
-            $context->lookupFunction('__compiler_utf8_strlen'),
-            $strPtr
-        );
+        return self::utf8LengthFromPtr($context, $str);
     }
 }
diff --git a/ext/mbstring/mb_strlen.php b/ext/mbstring/mb_strlen.php
index 1203311cf5a..b8ea34a2c74 100644
--- a/ext/mbstring/mb_strlen.php
+++ b/ext/mbstring/mb_strlen.php
@@ -8,6 +8,7 @@
 use PHPCompiler\Frame;
 use PHPCompiler\Func\Internal;
 use PHPCompiler\JIT\Context;
+use PHPCompiler\JIT\JitStringBuiltinArg;
 use PHPCompiler\JIT\Variable;
 use PHPCompiler\VM\Variable as VMVariable;
 use PHPLLVM\Value;
@@ -30,21 +31,23 @@ public function execute(Frame $frame): void
         if ($argc < 1 || $argc > 2) {
             throw new \LogicException('mb_strlen() requires one or two arguments');
         }
-        $strVar = $frame->calledArgs[0]->resolveIndirect();
-        if (VMVariable::TYPE_STRING !== $strVar->type) {
-            throw new \LogicException('mb_strlen() only supports strings in this compiler build');
-        }
+        $str = VmString::coerceStringBuiltinArg(
+            $frame->calledArgs[0],
+            'mb_strlen',
+            0,
+            'string'
+        );
         if (null === $frame->returnVar) {
             return;
         }
-        $str = $strVar->toString();
         $encoding = 'UTF-8';
         if (2 === $argc) {
-            $encVar = $frame->calledArgs[1]->resolveIndirect();
-            if (VMVariable::TYPE_STRING !== $encVar->type) {
-                throw new \LogicException('mb_strlen() encoding must be a string in this compiler build');
-            }
-            $encoding = $encVar->toString();
+            $encoding = VmString::coerceStringBuiltinArg(
+                $frame->calledArgs[1],
+                'mb_strlen',
+                1,
+                'encoding'
+            );
         }
         $frame->returnVar->int(self::lengthForEncoding($str, $encoding));
     }
@@ -55,29 +58,35 @@ public function call(Context $context, Variable ...$args): Value
         if ($argc < 1 || $argc > 2) {
             throw new \LogicException('mb_strlen() requires one or two arguments');
         }
+        if (1 === $argc && Variable::TYPE_STRING === $args[0]->type && null !== ($args[0]->compileTimeString ?? null)) {
+            return $context->constantFromInteger(
+                VmString::utf8CharLength($args[0]->compileTimeString),
+                'int64'
+            );
+        }
+
+        $str = JitStringBuiltinArg::lower($context, $args[0], 'mb_strlen', 0, 'string');
+
         if (1 === $argc) {
-            return JitMbStrlen::utf8Length($context, $args[0]);
+            return JitMbStrlen::utf8LengthFromPtr($context, $str);
         }
         if (Variable::TYPE_STRING !== $args[1]->type) {
             throw new \LogicException('mb_strlen() encoding must be a string in this compiler build');
         }
         $encoding = $args[1]->compileTimeString ?? null;
         if ('UTF-8' === $encoding) {
-            return JitMbStrlen::utf8Length($context, $args[0]);
+            return JitMbStrlen::utf8LengthFromPtr($context, $str);
         }
         if (null !== $encoding && 'ASCII' !== $encoding && '8BIT' !== $encoding) {
             throw new \LogicException(
                 'mb_strlen() JIT only supports UTF-8, ASCII, or 8BIT encoding literals in this compiler build'
             );
         }
-        if (Variable::TYPE_STRING !== $args[0]->type) {
-            throw new \LogicException('mb_strlen() only supports strings in this compiler build');
-        }
-        $argValue = $context->helper->loadValue($args[0]);
-        $offset = $context->structFieldIndex($argValue, 'length');
+
+        $offset = $context->structFieldIndex($str, 'length');
 
         return $context->builder->load(
-            $context->builder->structGep($argValue, $offset)
+            $context->builder->structGep($str, $offset)
         );
     }
 
diff --git a/test/compliance/MbStrlenJITTest.php b/test/compliance/MbStrlenJITTest.php
new file mode 100644
index 00000000000..b95831dddc2
--- /dev/null
+++ b/test/compliance/MbStrlenJITTest.php
@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPCompiler;
+
+require_once __DIR__.'/../BaseTest.php';
+
+/**
+ * @group llvm
+ */
+/** JIT compliance for mb_strlen(). */
+final class MbStrlenJITTest extends BaseTest
+{
+    protected static string $DIR = __DIR__;
+
+    public static function providePHPTests(): \Generator
+    {
+        yield 'mb_strlen_jit.phpt' => self::parsePHPT(
+            __DIR__.'/cases/stdlib/mb_strlen_jit.phpt',
+            'mb_strlen_jit.phpt'
+        );
+        yield 'mb_strlen_enum_typeerror_jit.phpt' => self::parsePHPT(
+            __DIR__.'/cases/stdlib/mb_strlen_enum_typeerror_jit.phpt',
+            'mb_strlen_enum_typeerror_jit.phpt'
+        );
+    }
+
+    public function setUp(): void
+    {
+        $this->BIN = realpath(__DIR__.'/../../bin/jit.php');
+        if (!LlvmToolchain::hasLibrary(dirname(__DIR__, 2))) {
+            $this->markTestSkipped(
+                'LLVM 9 toolchain not available. Run script/install-llvm9.sh or use the 22.04-dev Docker image.'
+            );
+        }
+    }
+}
diff --git a/test/compliance/MbStrlenVMTest.php b/test/compliance/MbStrlenVMTest.php
new file mode 100644
index 00000000000..513bccc81d4
--- /dev/null
+++ b/test/compliance/MbStrlenVMTest.php
@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPCompiler;
+
+require_once __DIR__.'/../BaseTest.php';
+
+/** VM compliance for mb_strlen(). */
+final class MbStrlenVMTest extends BaseTest
+{
+    protected static string $DIR = __DIR__;
+
+    public static function providePHPTests(): \Generator
+    {
+        yield 'mb_strlen.phpt' => self::parsePHPT(
+            __DIR__.'/cases/stdlib/mb_strlen.phpt',
+            'mb_strlen.phpt'
+        );
+        yield 'mb_strlen_enum_typeerror.phpt' => self::parsePHPT(
+            __DIR__.'/cases/stdlib/mb_strlen_enum_typeerror.phpt',
+            'mb_strlen_enum_typeerror.phpt'
+        );
+    }
+
+    public function setUp(): void
+    {
+        $this->BIN = realpath(__DIR__.'/../../bin/vm.php');
+    }
+}
diff --git a/test/compliance/cases/stdlib/mb_strlen_enum_typeerror.phpt b/test/compliance/cases/stdlib/mb_strlen_enum_typeerror.phpt
new file mode 100644
index 00000000000..638d55bfab2
--- /dev/null
+++ b/test/compliance/cases/stdlib/mb_strlen_enum_typeerror.phpt
@@ -0,0 +1,13 @@
+--TEST--
+stdlib mb_strlen() — backed enum case TypeError (#5873, ext/mbstring/mbstring.c)
+--FILE--
+<?php
+enum Es: string { case B = 'hi'; }
+try {
+    mb_strlen(Es::B);
+    echo "uncaught\n";
+} catch (TypeError $e) {
+    echo $e->getMessage(), "\n";
+}
+--EXPECT--
+mb_strlen(): Argument #1 ($string) must be of type string, Es given
diff --git a/test/compliance/cases/stdlib/mb_strlen_enum_typeerror_jit.phpt b/test/compliance/cases/stdlib/mb_strlen_enum_typeerror_jit.phpt
new file mode 100644
index 00000000000..2d778891896
--- /dev/null
+++ b/test/compliance/cases/stdlib/mb_strlen_enum_typeerror_jit.phpt
@@ -0,0 +1,13 @@
+--TEST--
+stdlib mb_strlen() JIT — backed enum case TypeError (#5873)
+--FILE--
+<?php
+enum Es: string { case B = 'hi'; }
+try {
+    mb_strlen(Es::B);
+    echo "uncaught\n";
+} catch (TypeError $e) {
+    echo $e->getMessage(), "\n";
+}
+--EXPECT--
+mb_strlen(): Argument #1 ($string) must be of type string, Es given
diff --git a/test/fixtures/aot/cases/mb_strlen_enum_typeerror.phpt b/test/fixtures/aot/cases/mb_strlen_enum_typeerror.phpt
new file mode 100644
index 00000000000..84311edb631
--- /dev/null
+++ b/test/fixtures/aot/cases/mb_strlen_enum_typeerror.phpt
@@ -0,0 +1,13 @@
+--TEST--
+AOT: mb_strlen() — backed enum case TypeError (#5873)
+--FILE--
+<?php
+enum Es: string { case B = 'hi'; }
+try {
+    mb_strlen(Es::B);
+    echo "uncaught\n";
+} catch (TypeError $e) {
+    echo $e->getMessage(), "\n";
+}
+--EXPECT--
+mb_strlen(): Argument #1 ($string) must be of type string, Es given
diff --git a/test/repro/maintainer_mb_strlen_enum.php b/test/repro/maintainer_mb_strlen_enum.php
new file mode 100644
index 00000000000..f1c49769814
--- /dev/null
+++ b/test/repro/maintainer_mb_strlen_enum.php
@@ -0,0 +1,7 @@
+<?php
+enum Es: string { case B = 'hi'; }
+try {
+    echo mb_strlen(Es::B);
+} catch (Throwable $e) {
+    echo get_class($e).': '.$e->getMessage();
+}