diff --git a/.github/workflows/back-end.yml b/.github/workflows/back-end.yml
index 8dce6a04..70188e06 100644
--- a/.github/workflows/back-end.yml
+++ b/.github/workflows/back-end.yml
@@ -76,6 +76,17 @@ jobs:
             uses: "ramsey/composer-install@v3"
             with:
                 dependency-versions: "${{ matrix.dependencies }}"
+        -
+            name: "Declare strict types"
+            if: "${{ matrix.dependencies == 'highest' }}"
+            run: |
+                echo "::group::Install slevomat/coding-standard"
+                composer config --no-plugins allow-plugins.dealerdirect/phpcodesniffer-composer-installer false
+                composer require --no-scripts --dev slevomat/coding-standard
+                echo "::endgroup::"
+                composer exec -- phpcbf --standard=vendor/slevomat/coding-standard/SlevomatCodingStandard \
+                    --sniffs=SlevomatCodingStandard.TypeHints.DeclareStrictTypes src/ \
+                    || test "$?" = 1 && exit 0
         -
             name: "Execute unit tests"
             run: "composer exec -- phpunit"
diff --git a/src/Fields/Field.php b/src/Fields/Field.php
index bd0bc370..6c63ed6f 100644
--- a/src/Fields/Field.php
+++ b/src/Fields/Field.php
@@ -519,7 +519,7 @@ public function resolveFormat(Request $request, Model $model): ?string
         $value = $this->resolveValue($request, $model);
 
         if (is_null($this->formatResolver)) {
-            return is_array($value) ? json_encode($value) : $value;
+            return is_array($value) ? json_encode($value) : (string) $value;
         }
 
         return call_user_func_array($this->formatResolver, [$request, $model, $value]);
diff --git a/src/Fields/Relation.php b/src/Fields/Relation.php
index 901ef458..520af659 100644
--- a/src/Fields/Relation.php
+++ b/src/Fields/Relation.php
@@ -679,7 +679,7 @@ public function mapRelated(Request $request, Model $model, Model $related): arra
      */
     public function modelUrl(Model $model): string
     {
-        return str_replace('{resourceModel}', $model->exists ? $model->getKey() : 'create', $this->getUri());
+        return str_replace('{resourceModel}', $model->exists ? (string) $model->getKey() : 'create', $this->getUri());
     }
 
     /**
@@ -906,7 +906,7 @@ public function parseQueryString(string $url): array
     {
         $query = parse_url($url, PHP_URL_QUERY);
 
-        parse_str($query, $result);
+        parse_str($query ?? '', $result);
 
         return array_filter($result, fn (string $key): bool => str_starts_with($key, $this->getRequestKey()), ARRAY_FILTER_USE_KEY);
     }
diff --git a/src/Fields/Repeater.php b/src/Fields/Repeater.php
index 876140c3..5130f811 100644
--- a/src/Fields/Repeater.php
+++ b/src/Fields/Repeater.php
@@ -207,7 +207,7 @@ public function buildOption(Request $request, Model $model): array
      */
     public function modelUrl(Model $model): string
     {
-        return str_replace('{resourceModel}', $model->exists ? $model->getKey() : 'create', $this->getUri());
+        return str_replace('{resourceModel}', $model->exists ? (string) $model->getKey() : 'create', $this->getUri());
     }
 
     /**
diff --git a/src/Resources/Resource.php b/src/Resources/Resource.php
index b13cde14..2317a2f0 100644
--- a/src/Resources/Resource.php
+++ b/src/Resources/Resource.php
@@ -315,7 +315,7 @@ public function modelUrl(Model $model): string
      */
     public function modelTitle(Model $model): string
     {
-        return $model->getKey();
+        return (string) $model->getKey();
     }
 
     /**
diff --git a/tests/Http/RelationControllerTest.php b/tests/Http/RelationControllerTest.php
index 923eff0d..41706441 100644
--- a/tests/Http/RelationControllerTest.php
+++ b/tests/Http/RelationControllerTest.php
@@ -35,9 +35,11 @@ protected function setUp(): void
         );
     }
 
+    #[\PHPUnit\Framework\Attributes\WithoutErrorHandler]
     public function test_relation_controller_handles_index(): void
     {
         $this->actingAs($this->admin)
+            ->withoutExceptionHandling()
             ->get('/root/resources/users/'.$this->admin->getKey().'/fields/uploads')
             ->assertOk()
             ->assertViewIs('root::resources.index')