diff --git a/src/Psalm/Checker/Statements/Expression/Fetch/ArrayFetchChecker.php b/src/Psalm/Checker/Statements/Expression/Fetch/ArrayFetchChecker.php
index e7cc03cb5..61a3a5a5d 100644
--- a/src/Psalm/Checker/Statements/Expression/Fetch/ArrayFetchChecker.php
+++ b/src/Psalm/Checker/Statements/Expression/Fetch/ArrayFetchChecker.php
@@ -380,10 +380,12 @@ class ArrayFetchChecker
                             $type->type_params[0] = $offset_type;
                         }
                     } elseif (!$type->type_params[0]->isEmpty()) {
-                        if (!TypeChecker::isContainedBy(
+                        if ((!TypeChecker::isContainedBy(
                             $project_checker->codebase,
                             $offset_type,
-                            $type->type_params[0],
+                            $type->type_params[0]->isMixed()
+                                ? new Type\Union([ new TInt, new TString ])
+                                : $type->type_params[0],
                             true,
                             $offset_type->ignore_falsable_issues,
                             $has_scalar_match,
@@ -391,7 +393,8 @@ class ArrayFetchChecker
                             $type_coerced_from_mixed,
                             $to_string_cast,
                             $type_coerced_from_scalar
-                        ) && !$type_coerced_from_scalar
+                        ) && !$type_coerced_from_scalar)
+                            || $to_string_cast
                         ) {
                             $expected_offset_types[] = $type->type_params[0]->getId();
                         } else {
diff --git a/tests/ArrayAccessTest.php b/tests/ArrayAccessTest.php
index 79e2cfbf7..c12133bc7 100644
--- a/tests/ArrayAccessTest.php
+++ b/tests/ArrayAccessTest.php
@@ -149,6 +149,14 @@ class ArrayAccessTest extends TestCase
                 'assertions' => [],
                 'error_levels' => ['MixedArgument'],
             ],
+            'mixedKeyMixedOffset' => [
+                '<?php
+                    function example(array $x, $y) : void {
+                        echo $x[$y];
+                    }',
+                'assertions' => [],
+                'error_levels' => ['MixedArgument', 'MixedArrayOffset', 'MissingParamType'],
+            ],
         ];
     }
 
@@ -250,6 +258,13 @@ class ArrayAccessTest extends TestCase
                     }',
                 'error_message' => 'InvalidArrayOffset',
             ],
+            'mixedKeyStdClassOffset' => [
+                '<?php
+                    function example(array $y) : void {
+                        echo $y[new stdClass()];
+                    }',
+                'error_message' => 'InvalidArrayOffset',
+            ],
         ];
     }
 }