diff --git a/src/libfetchers/github.cc b/src/libfetchers/github.cc
index d62a7482e..267e8607f 100644
--- a/src/libfetchers/github.cc
+++ b/src/libfetchers/github.cc
@@ -433,9 +433,15 @@ struct GitLabInputScheme : GitArchiveInputScheme
                 store->toRealPath(
                     downloadFile(store, url, "source", headers).storePath)));
 
-        return RefInfo {
-            .rev = Hash::parseAny(std::string(json[0]["id"]), HashAlgorithm::SHA1)
-        };
+        if (json.is_array() && json.size() == 1 && json[0]["id"] != nullptr) {
+          return RefInfo {
+              .rev = Hash::parseAny(std::string(json[0]["id"]), HashAlgorithm::SHA1)
+          };
+        } if (json.is_array() && json.size() == 0) {
+            throw Error("No commits returned by GitLab API -- does the git ref really exist?");
+        } else {
+            throw Error("Unexpected response received from GitLab: %s", json);
+        }
     }
 
     DownloadUrl getDownloadUrl(const Input & input) const override