diff --git a/src/Api/Encoder.php b/src/Api/Encoder.php
index 81e6862..eab97e0 100644
--- a/src/Api/Encoder.php
+++ b/src/Api/Encoder.php
@@ -104,7 +104,11 @@ public function getFormat(ImageInterface $image): string
     {
         $fm = (string) $this->getParam('fm');
         if ($fm) {
-            return array_key_exists($fm, static::supportedFormats()) ? $fm : 'jpg';
+            if (!array_key_exists($fm, static::supportedFormats())) {
+                throw new \InvalidArgumentException("Invalid format provided: {$fm}");
+            }
+
+            return $fm;
         }
 
         $mediaType = MediaType::tryFrom($image->origin()->mediaType());
diff --git a/tests/Api/EncoderTest.php b/tests/Api/EncoderTest.php
index 025d10a..d437bcd 100644
--- a/tests/Api/EncoderTest.php
+++ b/tests/Api/EncoderTest.php
@@ -123,7 +123,6 @@ public function testGetFormat(): void
 
         $this->assertSame('jpg', $this->encoder->setParams(['fm' => ''])->getFormat($this->getImageByMimeType('image/jpeg')));
         $this->assertSame('png', $this->encoder->setParams(['fm' => ''])->getFormat($this->getImageByMimeType('image/png')));
-        $this->assertSame('jpg', $this->encoder->setParams(['fm' => 'invalid'])->getFormat($this->getImageByMimeType('image/png')));
 
         if (function_exists('imagecreatefromwebp')) {
             $this->assertSame('webp', $this->encoder->setParams(['fm' => null])->getFormat($this->getImageByMimeType('image/webp')));
@@ -136,6 +135,13 @@ public function testGetFormat(): void
         }
     }
 
+    public function testGetFormatThrowsExceptionForInvalidFormat(): void
+    {
+        $this->expectException(\InvalidArgumentException::class);
+        $this->expectExceptionMessage('Invalid format provided: invalid');
+        $this->encoder->setParams(['fm' => 'invalid'])->getFormat($this->getImageByMimeType('image/png'));
+    }
+
     protected function getImageByMimeType(string $mimeType): ImageInterface
     {
         return \Mockery::mock(ImageInterface::class, function ($mock) use ($mimeType) {