diff --git a/src/Internal/PsrStreamBody.php b/src/Internal/PsrStreamBody.php
index 94bb46b..a3fb879 100644
--- a/src/Internal/PsrStreamBody.php
+++ b/src/Internal/PsrStreamBody.php
@@ -22,7 +22,7 @@ public function getContent(): ReadableStream
 
     public function getContentLength(): ?int
     {
-        return $this->stream->getSize() ?? -1;
+        return $this->stream->getSize();
     }
 
     public function getContentType(): ?string
diff --git a/test/Internal/PsrStreamBodyTest.php b/test/Internal/PsrStreamBodyTest.php
index 31f32a8..6c9e60d 100644
--- a/test/Internal/PsrStreamBodyTest.php
+++ b/test/Internal/PsrStreamBodyTest.php
@@ -15,7 +15,7 @@ class PsrStreamBodyTest extends TestCase
     /**
      * @dataProvider providerBodyLength
      */
-    public function testGetBodyLengthReturnsValueFromStream(?int $size, int $expectedSize): void
+    public function testGetBodyLengthReturnsValueFromStream(?int $size, ?int $expectedSize): void
     {
         $stream = $this->createMock(StreamInterface::class);
         $stream->method('getSize')->willReturn($size);
@@ -30,7 +30,7 @@ public function providerBodyLength(): array
         return [
             'Stream provides zero size' => [0, 0],
             'Stream provides positive size' => [1, 1],
-            'Stream doesn\'t provide its size' => [null, -1],
+            'Stream doesn\'t provide its size' => [null, null],
         ];
     }