diff --git a/lix/libfetchers/github.cc b/lix/libfetchers/github.cc
index 4b36c066c..7a4362c68 100644
--- a/lix/libfetchers/github.cc
+++ b/lix/libfetchers/github.cc
@@ -150,8 +150,15 @@ struct GitArchiveInputScheme : InputScheme
         auto ref = input.getRef();
         auto rev = input.getRev();
         auto path = owner + "/" + repo;
-        assert(!(ref && rev));
-        if (ref) path += "/" + *ref;
+        if (ref && rev) {
+            throw Error(
+                "input '%s:%s/%s' has both ref (%s) and rev (%s), which is not allowed",
+                schemeType(), owner, repo, *ref, rev->gitRev()
+            );
+        }
+        if (ref) {
+            path += "/" + *ref;
+        }
         if (rev) {
             path += "/" + rev->to_string(HashFormat::Base16, false);
         }
diff --git a/tests/functional2/lang/builtins.fetchtree/eval-fail-github-ref-rev.err.exp b/tests/functional2/lang/builtins.fetchtree/eval-fail-github-ref-rev.err.exp
new file mode 100644
index 000000000..264342eb9
--- /dev/null
+++ b/tests/functional2/lang/builtins.fetchtree/eval-fail-github-ref-rev.err.exp
@@ -0,0 +1,9 @@
+error:
+       … while calling the 'fetchTree' builtin
+         at /pwd/in.nix:3:1:
+            2| # https://git.lix.systems/lix-project/lix/issues/1133
+            3| builtins.fetchTree {
+             | ^
+            4|   type = "github";
+
+       error: input 'github:nixos/nixpkgs' has both ref (nixpkgs-unstable) and rev (e4bae1bd10c9c57b2cf517953ab70060a828ee6f), which is not allowed
diff --git a/tests/functional2/lang/builtins.fetchtree/in-github-ref-rev.nix b/tests/functional2/lang/builtins.fetchtree/in-github-ref-rev.nix
new file mode 100644
index 000000000..eb381dce0
--- /dev/null
+++ b/tests/functional2/lang/builtins.fetchtree/in-github-ref-rev.nix
@@ -0,0 +1,9 @@
+# tests that this produces a proper error, as it didn't before
+# https://git.lix.systems/lix-project/lix/issues/1133
+builtins.fetchTree {
+  type = "github";
+  owner = "nixos";
+  repo = "nixpkgs";
+  ref = "nixpkgs-unstable";
+  rev = "e4bae1bd10c9c57b2cf517953ab70060a828ee6f";
+}