diff --git a/shelfmark/download/http.py b/shelfmark/download/http.py
index 106c60c2..079f16fa 100644
--- a/shelfmark/download/http.py
+++ b/shelfmark/download/http.py
@@ -1104,5 +1104,7 @@ def get_absolute_url(base_url: str, url: str) -> str:
     parsed = urlparse(url)
     base = urlparse(base_url)
     if not parsed.netloc or not parsed.scheme:
-        parsed = parsed._replace(netloc=base.netloc, scheme=base.scheme)
+        parsed = parsed._replace(
+            netloc=parsed.netloc or base.netloc, scheme=parsed.scheme or base.scheme
+        )
     return parsed.geturl()
diff --git a/tests/download/test_http_absolute_url.py b/tests/download/test_http_absolute_url.py
new file mode 100644
index 00000000..32f65029
--- /dev/null
+++ b/tests/download/test_http_absolute_url.py
@@ -0,0 +1,17 @@
+"""Tests for resolving a scraped link against the page it came from."""
+
+
+def test_get_absolute_url_keeps_a_protocol_relative_host():
+    import shelfmark.download.http as http
+
+    result = http.get_absolute_url("https://annas-archive.org/md5/abc", "//cdn.example.org/f.epub")
+
+    assert result == "https://cdn.example.org/f.epub"
+
+
+def test_get_absolute_url_resolves_a_relative_path_against_the_base():
+    import shelfmark.download.http as http
+
+    result = http.get_absolute_url("https://annas-archive.org/md5/abc", "/slow_download/abc/0/1")
+
+    assert result == "https://annas-archive.org/slow_download/abc/0/1"