diff --git a/test/src/Grant/GrantTestCase.php b/test/src/Grant/GrantTestCase.php
index 5451b331..125c66ce 100644
--- a/test/src/Grant/GrantTestCase.php
+++ b/test/src/Grant/GrantTestCase.php
@@ -65,7 +65,7 @@ public function testGetAccessToken($grant, array $params = [])
             ->shouldReceive('getHeader')
             ->once()
             ->with('content-type')
-            ->andReturn('application/json');
+            ->andReturn(['application/json']);
 
         /** @var ClientInterface & MockInterface $client */
         $client = Mockery::spy(ClientInterface::class)->makePartial();
diff --git a/test/src/Provider/AbstractProviderTest.php b/test/src/Provider/AbstractProviderTest.php
index b9ebf6f1..93cf97a7 100644
--- a/test/src/Provider/AbstractProviderTest.php
+++ b/test/src/Provider/AbstractProviderTest.php
@@ -213,7 +213,7 @@ public function testGetUserProperties($name = null, $email = null, $id = null)
             ->shouldReceive('getHeader')
             ->once()
             ->with('content-type')
-            ->andReturn('application/json');
+            ->andReturn(['application/json']);
 
 
         $client = Mockery::spy(ClientInterface::class, [
@@ -262,7 +262,7 @@ public function testGetUserPropertiesThrowsExceptionWhenNonJsonResponseIsReceive
         $response
             ->shouldReceive('getHeader')
             ->with('content-type')
-            ->andReturn('text/html');
+            ->andReturn(['text/html']);
 
         $client = Mockery::mock(ClientInterface::class, [
             'send' => $response,
@@ -378,7 +378,7 @@ public function testPkceMethod($pkceMethod, $pkceCode, $expectedChallenge)
             ->shouldReceive('getHeader')
             ->once()
             ->with('content-type')
-            ->andReturn('application/json');
+            ->andReturn(['application/json']);
 
         $client = Mockery::spy(ClientInterface::class, [
             'send' => $response,
@@ -474,7 +474,7 @@ public function testErrorResponsesCanBeCustomizedAtTheProvider()
             ->shouldReceive('getHeader')
             ->once()
             ->with('content-type')
-            ->andReturn('application/json');
+            ->andReturn(['application/json']);
 
         $client = Mockery::spy(ClientInterface::class, [
             'send' => $response,
@@ -532,7 +532,7 @@ public function testClientErrorTriggersProviderException()
         $response
             ->shouldReceive('getHeader')
             ->with('content-type')
-            ->andReturn('application/json');
+            ->andReturn(['application/json']);
 
         $client = Mockery::mock(ClientInterface::class);
         $client
@@ -579,7 +579,7 @@ public function testAuthenticatedRequestAndResponse()
         $response
             ->shouldReceive('getHeader')
             ->with('content-type')
-            ->andReturn('application/json');
+            ->andReturn(['application/json']);
 
         $client = Mockery::mock(ClientInterface::class);
         $client
@@ -647,7 +647,7 @@ public function testGetAccessToken($method)
             ->shouldReceive('getHeader')
             ->once()
             ->with('content-type')
-            ->andReturn('application/json');
+            ->andReturn(['application/json']);
 
         $client = Mockery::spy(ClientInterface::class, [
             'send' => $response,
@@ -686,7 +686,7 @@ public function testGetAccessTokenWithNonJsonResponse()
         $response
             ->shouldReceive('getHeader')
             ->with('content-type')
-            ->andReturn('text/plain');
+            ->andReturn(['text/plain']);
 
         $client = Mockery::mock(ClientInterface::class, [
             'send' => $response,
@@ -745,7 +745,7 @@ public function testParseResponse($body, $type, $parsed, $statusCode = 200)
         $response
             ->shouldReceive('getHeader')
             ->with('content-type')
-            ->andReturn($type);
+            ->andReturn([$type]);
 
         $method = $this->getMethod(AbstractProvider::class, 'parseResponse');
         $result = $method->invoke($this->getMockProvider(), $response);