diff --git a/lib/private/Image.php b/lib/private/Image.php
index ca5058e6c65e5..396246125604f 100644
--- a/lib/private/Image.php
+++ b/lib/private/Image.php
@@ -166,7 +166,9 @@ public function show(?string $mimeType = null): bool {
 		if ($mimeType === null) {
 			$mimeType = $this->mimeType();
 		}
-		header('Content-Type: ' . ($mimeType ?? ''));
+		if ($mimeType !== null) {
+			header('Content-Type: ' . $mimeType);
+		}
 		return $this->_output(null, $mimeType);
 	}