diff --git a/src/Psalm/Checker/Statements/Expression/Fetch/ArrayFetchChecker.php b/src/Psalm/Checker/Statements/Expression/Fetch/ArrayFetchChecker.php
index 61a3a5a5d..7fe8e4618 100644
--- a/src/Psalm/Checker/Statements/Expression/Fetch/ArrayFetchChecker.php
+++ b/src/Psalm/Checker/Statements/Expression/Fetch/ArrayFetchChecker.php
@@ -441,6 +441,8 @@ class ArrayFetchChecker
                         }
                     }
                 } else {
+                    $generic_key_type = $type->getGenericKeyType();
+
                     if ($key_value !== null) {
                         if (isset($type->properties[$key_value]) || $replacement_type) {
                             $has_valid_offset = true;
@@ -492,10 +494,12 @@ class ArrayFetchChecker
 
                             $array_access_type = Type::getMixed();
                         }
-                    } elseif (TypeChecker::isContainedBy(
+                    } elseif ((TypeChecker::isContainedBy(
                         $codebase,
                         $offset_type,
-                        $type->getGenericKeyType(),
+                        $generic_key_type->isMixed()
+                            ? new Type\Union([ new TInt, new TString ])
+                            : $generic_key_type,
                         true,
                         $offset_type->ignore_falsable_issues,
                         $has_scalar_match,
@@ -505,7 +509,8 @@ class ArrayFetchChecker
                         $type_coerced_from_scalar
                     )
                     || $type_coerced_from_scalar
-                    || $in_assignment
+                    || $in_assignment)
+                    && !$to_string_cast
                     ) {
                         if ($replacement_type) {
                             $generic_params = Type::combineUnionTypes(
@@ -514,7 +519,7 @@ class ArrayFetchChecker
                             );
 
                             $new_key_type = Type::combineUnionTypes(
-                                $type->getGenericKeyType(),
+                                $generic_key_type,
                                 $offset_type
                             );
 
@@ -552,7 +557,7 @@ class ArrayFetchChecker
                         $has_valid_offset = true;
                     } else {
                         if (!$inside_isset || $type->sealed) {
-                            $expected_offset_types[] = (string)$type->getGenericKeyType()->getId();
+                            $expected_offset_types[] = (string)$generic_key_type->getId();
                         }
 
                         $array_access_type = Type::getMixed();
diff --git a/tests/ArrayAccessTest.php b/tests/ArrayAccessTest.php
index c12133bc7..260205728 100644
--- a/tests/ArrayAccessTest.php
+++ b/tests/ArrayAccessTest.php
@@ -265,6 +265,18 @@ class ArrayAccessTest extends TestCase
                     }',
                 'error_message' => 'InvalidArrayOffset',
             ],
+            'toStringOffset' => [
+                '<?php
+                    class Foo {
+                        public function __toString() {
+                            return "Foo";
+                        }
+                    }
+
+                    $a = ["Foo" => "bar"];
+                    echo $a[new Foo];',
+                'error_message' => 'InvalidArrayOffset',
+            ],
         ];
     }
 }