diff --git a/src/Type/ArrayType.php b/src/Type/ArrayType.php
index 01ca997eee..c3fc5050b1 100644
--- a/src/Type/ArrayType.php
+++ b/src/Type/ArrayType.php
@@ -21,6 +21,7 @@
 use PHPStan\Type\Constant\ConstantIntegerType;
 use PHPStan\Type\Constant\ConstantStringType;
 use PHPStan\Type\Generic\TemplateMixedType;
+use PHPStan\Type\Generic\TemplateStrictMixedType;
 use PHPStan\Type\Generic\TemplateTypeMap;
 use PHPStan\Type\Generic\TemplateTypeVariance;
 use PHPStan\Type\Traits\MaybeCallableTypeTrait;
@@ -50,6 +51,10 @@ public function __construct(Type $keyType, private Type $itemType)
 		if ($keyType->describe(VerbosityLevel::value()) === '(int|string)') {
 			$keyType = new MixedType();
 		}
+		if ($keyType instanceof StrictMixedType && !$keyType instanceof TemplateStrictMixedType) {
+			$keyType = new UnionType([new StringType(), new IntegerType()]);
+		}
+
 		$this->keyType = $keyType;
 	}
 
diff --git a/tests/PHPStan/Rules/Functions/CallToFunctionParametersRuleTest.php b/tests/PHPStan/Rules/Functions/CallToFunctionParametersRuleTest.php
index c105b2fe64..768d75551f 100644
--- a/tests/PHPStan/Rules/Functions/CallToFunctionParametersRuleTest.php
+++ b/tests/PHPStan/Rules/Functions/CallToFunctionParametersRuleTest.php
@@ -21,12 +21,14 @@ class CallToFunctionParametersRuleTest extends RuleTestCase
 
 	private bool $checkExplicitMixed = false;
 
+	private bool $checkImplicitMixed = false;
+
 	protected function getRule(): Rule
 	{
 		$broker = $this->createReflectionProvider();
 		return new CallToFunctionParametersRule(
 			$broker,
-			new FunctionCallParametersCheck(new RuleLevelHelper($broker, true, false, true, $this->checkExplicitMixed, false, true, false), new NullsafeCheck(), new PhpVersion(80000), new UnresolvableTypeHelper(), new PropertyReflectionFinder(), true, true, true, true, true),
+			new FunctionCallParametersCheck(new RuleLevelHelper($broker, true, false, true, $this->checkExplicitMixed, $this->checkImplicitMixed, true, false), new NullsafeCheck(), new PhpVersion(80000), new UnresolvableTypeHelper(), new PropertyReflectionFinder(), true, true, true, true, true),
 		);
 	}
 
@@ -1903,4 +1905,11 @@ public function testBug11759(): void
 		$this->analyse([__DIR__ . '/data/bug-11759.php'], []);
 	}
 
+	public function testBug12051(): void
+	{
+		$this->checkExplicitMixed = true;
+		$this->checkImplicitMixed = true;
+		$this->analyse([__DIR__ . '/data/bug-12051.php'], []);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Functions/data/bug-12051.php b/tests/PHPStan/Rules/Functions/data/bug-12051.php
new file mode 100644
index 0000000000..d271c7bf52
--- /dev/null
+++ b/tests/PHPStan/Rules/Functions/data/bug-12051.php
@@ -0,0 +1,15 @@
+<?php
+
+namespace Bug12051;
+
+/** @param array<int|string, mixed> $a */
+function foo($a): void {
+	print "ok\n";
+}
+
+/**
+ * @param array<mixed> $a
+ */
+function bar($a): void {
+	foo($a);
+}