diff --git a/swh/lister/nixguix/lister.py b/swh/lister/nixguix/lister.py
--- a/swh/lister/nixguix/lister.py
+++ b/swh/lister/nixguix/lister.py
@@ -129,8 +129,7 @@
     url = urls[index]
 
     try:
-        is_tar = _is_tarball(url)
-        return is_tar, urls[0]
+        return _is_tarball(url), urls[0]
     except IndexError:
         if request is None:
             raise ArtifactNatureUndetected(
@@ -285,15 +284,20 @@
                 )
             elif artifact_type == "url":
                 # It's either a tarball or a file
-                urls = artifact.get("urls")
-                if not urls:
+                origin_urls = artifact.get("urls")
+                if not origin_urls:
                     # Nothing to fetch
                     logger.warning("Skipping url <%s>: empty artifact", artifact)
                     continue
 
-                assert urls is not None
+                assert origin_urls is not None
+
+                # Deal with urls with empty scheme (basic fallback to http)
+                urls = []
+                for url in origin_urls:
+                    urlparsed = urlparse(url)
+                    urls.append(f"http://{url}" if urlparsed.scheme == "" else url)
 
-                # FIXME: T3294: Fix missing scheme in urls
                 origin, *fallback_urls = urls
 
                 integrity = artifact.get("integrity")
diff --git a/swh/lister/nixguix/tests/data/nixpkgs-swh_sources.json b/swh/lister/nixguix/tests/data/nixpkgs-swh_sources.json
--- a/swh/lister/nixguix/tests/data/nixpkgs-swh_sources.json
+++ b/swh/lister/nixguix/tests/data/nixpkgs-swh_sources.json
@@ -29,6 +29,13 @@
       ],
       "integrity": "sha256-bss09x9yOnuW+Q5BHHjf8nNcCNxCKMdl9/2/jKSFcrQ="
     },
+    {
+      "type": "url",
+      "urls": [
+        "www.roudoudou.com/export/cpc/rasm/rasm_v0117_src.zip"
+      ],
+      "integrity": "sha256-wAEswtkl3ulAw3zq4perrGS6Wlww5XXnQYsEAoYT9fI="
+    },
     {
       "type": "url",
       "urls": [
diff --git a/swh/lister/nixguix/tests/test_lister.py b/swh/lister/nixguix/tests/test_lister.py
--- a/swh/lister/nixguix/tests/test_lister.py
+++ b/swh/lister/nixguix/tests/test_lister.py
@@ -146,7 +146,7 @@
         assert origin == url
 
 
-def test_lister_nixguix(datadir, swh_scheduler, requests_mock):
+def test_lister_nixguix_ok(datadir, swh_scheduler, requests_mock):
     """NixGuixLister should list all origins per visit type"""
     url = "https://nix-community.github.io/nixpkgs-swh/sources-unstable.json"
     origin_upstream = "https://github.com/NixOS/nixpkgs"