diff --git a/apps/files_sharing/lib/Controller/PublicPreviewController.php b/apps/files_sharing/lib/Controller/PublicPreviewController.php
index 9056161af84fe..f51b10688f240 100644
--- a/apps/files_sharing/lib/Controller/PublicPreviewController.php
+++ b/apps/files_sharing/lib/Controller/PublicPreviewController.php
@@ -59,12 +59,14 @@ protected function isPasswordProtected(): bool {
 		return $this->share->getPassword() !== null;
 	}
 
-
 	/**
-	 * Get a preview for a shared file
+	 * Get a preview for a public share.
+	 *
+	 * For shares pointing to a single file, the $file parameter is ignored and may be empty.
+	 * For folder shares, $file must be the relative path to a file inside the shared folder.
 	 *
 	 * @param string $token Token of the share
-	 * @param string $file File in the share
+	 * @param string $file Relative path to a file inside a shared folder; ignored for single-file shares
 	 * @param int $x Width of the preview
 	 * @param int $y Height of the preview
 	 * @param bool $a Whether to not crop the preview
@@ -122,19 +124,22 @@ public function getPreview(
 		try {
 			$node = $share->getNode();
 			if ($node instanceof Folder) {
-				$file = $node->get($file);
+				if ($file === '') {
+					return new DataResponse([], Http::STATUS_BAD_REQUEST);
+				}
+				$fileNode = $node->get($file);
 			} else {
-				$file = $node;
+				$fileNode = $node;
 			}
 
-			$f = $this->previewManager->getPreview($file, $x, $y, !$a);
+			$f = $this->previewManager->getPreview($fileNode, $x, $y, !$a);
 			$response = new FileDisplayResponse($f, Http::STATUS_OK, ['Content-Type' => $f->getMimeType()]);
 			$response->cacheFor($cacheForSeconds);
 			return $response;
 		} catch (NotFoundException $e) {
 			// If we have no preview enabled, we can redirect to the mime icon if any
-			if ($mimeFallback) {
-				if ($url = $this->mimeIconProvider->getMimeIconUrl($file->getMimeType())) {
+			if ($mimeFallback && isset($fileNode) && !($fileNode instanceof Folder)) {
+				if ($url = $this->mimeIconProvider->getMimeIconUrl($fileNode->getMimeType())) {
 					return new RedirectResponse($url);
 				}
 			}