diff --git a/lib/private/Preview/Generator.php b/lib/private/Preview/Generator.php
index 4a7341896ef6a..c0c55a44bb1d1 100644
--- a/lib/private/Preview/Generator.php
+++ b/lib/private/Preview/Generator.php
@@ -535,6 +535,9 @@ private function generatePreview(
 			self::unguardWithSemaphore($sem);
 		}
 
+		if (!$preview->valid() || $preview->dataMimeType() === null) {
+			throw new \InvalidArgumentException('Preview generation failed: invalid or null MIME type');
+		}
 
 		$path = $this->generatePath($width, $height, $crop, false, $preview->dataMimeType(), $prefix);
 		try {