diff --git a/gallery_dl/downloader/http.py b/gallery_dl/downloader/http.py
index 94aa8f33..165f7898 100644
--- a/gallery_dl/downloader/http.py
+++ b/gallery_dl/downloader/http.py
@@ -486,7 +486,8 @@ MIME_TYPES = {
 
 
 def _signature_html(s):
-    return b"<!doctype html".startswith(s[:14].lower())
+    s = s[:14].lstrip()
+    return s and b"<!doctype html".startswith(s.lower())
 
 
 # https://en.wikipedia.org/wiki/List_of_file_signatures
diff --git a/test/test_downloader.py b/test/test_downloader.py
index a1acfdd9..3e5bf84a 100644
--- a/test/test_downloader.py
+++ b/test/test_downloader.py
@@ -370,6 +370,7 @@ SAMPLES = {
     ("heic", b"????ftypheix"),
     ("svg" , b"<?xml"),
     ("html", b"<!DOCTYPE html><html>...</html>"),
+    ("html", b"  \n  \n\r\t\n  <!DOCTYPE html><html>...</html>"),
     ("ico" , b"\x00\x00\x01\x00"),
     ("cur" , b"\x00\x00\x02\x00"),
     ("psd" , b"8BPS"),
