diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index e5eeea520dc..84e7f5c02ee 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -364,6 +364,10 @@ static RegisterPrimOp primop_fetchGit({
           A Boolean parameter that specifies whether submodules should be
           checked out. Defaults to `false`.
 
+        - shallow\
+          A Boolean parameter that specifies whether fetching a shallow clone
+          is allowed. Defaults to `false`.
+
         - allRefs\
           Whether to fetch all refs of the repository. With this argument being
           true, it's possible to load a `rev` from *any* `ref` (by default only
diff --git a/src/libfetchers/git.cc b/src/libfetchers/git.cc
index 35fdf807afd..7d01aaa7a01 100644
--- a/src/libfetchers/git.cc
+++ b/src/libfetchers/git.cc
@@ -574,7 +574,7 @@ struct GitInputScheme : InputScheme
         bool isShallow = chomp(runProgram("git", true, { "-C", repoDir, "--git-dir", gitDir, "rev-parse", "--is-shallow-repository" })) == "true";
 
         if (isShallow && !shallow)
-            throw Error("'%s' is a shallow Git repository, but a non-shallow repository is needed", actualUrl);
+            throw Error("'%s' is a shallow Git repository, but shallow repositories are only allowed when `shallow = true;` is specified.", actualUrl);
 
         // FIXME: check whether rev is an ancestor of ref.