diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 852e8aa11..6fcb2917c 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -158,7 +158,7 @@ string showType(ValueType type)
 {
     switch (type) {
         case tInt: return "an integer";
-        case tBool: return "a boolean";
+        case tBool: return "a Boolean";
         case tString: return "a string";
         case tPath: return "a path";
         case tNull: return "null";
diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index a9becddd9..2b3f1b76d 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -58,8 +58,10 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
             state.forceValue(*attr.value);
             if (attr.value->type == tString)
                 attrs.emplace(attr.name, attr.value->string.s);
+            else if (attr.value->type == tBool)
+                attrs.emplace(attr.name, attr.value->boolean);
             else
-                throw TypeError("fetchTree argument '%s' is %s while a string is expected",
+                throw TypeError("fetchTree argument '%s' is %s while a string or Boolean is expected",
                     attr.name, showType(*attr.value));
         }
 
diff --git a/src/libstore/fetchers/attrs.cc b/src/libstore/fetchers/attrs.cc
index 83b6d9164..40c02de42 100644
--- a/src/libstore/fetchers/attrs.cc
+++ b/src/libstore/fetchers/attrs.cc
@@ -14,6 +14,8 @@ Attrs jsonToAttrs(const nlohmann::json & json)
             attrs.emplace(i.key(), i.value().get<int64_t>());
         else if (i.value().is_string())
             attrs.emplace(i.key(), i.value().get<std::string>());
+        else if (i.value().is_boolean())
+            attrs.emplace(i.key(), i.value().get<bool>());
         else
             throw Error("unsupported input attribute type in lock file");
     }
@@ -29,6 +31,8 @@ nlohmann::json attrsToJson(const Attrs & attrs)
             json[attr.first] = *v;
         } else if (auto v = std::get_if<std::string>(&attr.second)) {
             json[attr.first] = *v;
+        } else if (auto v = std::get_if<Explicit<bool>>(&attr.second)) {
+            json[attr.first] = v->t;
         } else abort();
     }
     return json;
@@ -40,7 +44,7 @@ std::optional<std::string> maybeGetStrAttr(const Attrs & attrs, const std::strin
     if (i == attrs.end()) return {};
     if (auto v = std::get_if<std::string>(&i->second))
         return *v;
-    throw Error("input attribute '%s' is not a string", name);
+    throw Error("input attribute '%s' is not a string %s", name, attrsToJson(attrs).dump());
 }
 
 std::string getStrAttr(const Attrs & attrs, const std::string & name)
@@ -57,7 +61,7 @@ std::optional<int64_t> maybeGetIntAttr(const Attrs & attrs, const std::string &
     if (i == attrs.end()) return {};
     if (auto v = std::get_if<int64_t>(&i->second))
         return *v;
-    throw Error("input attribute '%s' is not a string", name);
+    throw Error("input attribute '%s' is not an integer", name);
 }
 
 int64_t getIntAttr(const Attrs & attrs, const std::string & name)
@@ -68,4 +72,21 @@ int64_t getIntAttr(const Attrs & attrs, const std::string & name)
     return *s;
 }
 
+std::optional<bool> maybeGetBoolAttr(const Attrs & attrs, const std::string & name)
+{
+    auto i = attrs.find(name);
+    if (i == attrs.end()) return {};
+    if (auto v = std::get_if<int64_t>(&i->second))
+        return *v;
+    throw Error("input attribute '%s' is not a Boolean", name);
+}
+
+bool getBoolAttr(const Attrs & attrs, const std::string & name)
+{
+    auto s = maybeGetBoolAttr(attrs, name);
+    if (!s)
+        throw Error("input attribute '%s' is missing", name);
+    return *s;
+}
+
 }
diff --git a/src/libstore/fetchers/attrs.hh b/src/libstore/fetchers/attrs.hh
index b7f98d71b..2c9e772d2 100644
--- a/src/libstore/fetchers/attrs.hh
+++ b/src/libstore/fetchers/attrs.hh
@@ -8,7 +8,14 @@
 
 namespace nix::fetchers {
 
-typedef std::variant<std::string, int64_t> Attr;
+/* Wrap bools to prevent string literals (i.e. 'char *') from being
+   cast to a bool in Attr. */
+template<typename T>
+struct Explicit {
+    T t;
+};
+
+typedef std::variant<std::string, int64_t, Explicit<bool>> Attr;
 typedef std::map<std::string, Attr> Attrs;
 
 Attrs jsonToAttrs(const nlohmann::json & json);
@@ -23,4 +30,8 @@ std::optional<int64_t> maybeGetIntAttr(const Attrs & attrs, const std::string &
 
 int64_t getIntAttr(const Attrs & attrs, const std::string & name);
 
+std::optional<bool> maybeGetBoolAttr(const Attrs & attrs, const std::string & name);
+
+bool getBoolAttr(const Attrs & attrs, const std::string & name);
+
 }
diff --git a/src/libstore/fetchers/git.cc b/src/libstore/fetchers/git.cc
index 179a5fb83..c5d4f019c 100644
--- a/src/libstore/fetchers/git.cc
+++ b/src/libstore/fetchers/git.cc
@@ -83,6 +83,7 @@ struct GitInput : Input
     ParsedURL url;
     std::optional<std::string> ref;
     std::optional<Hash> rev;
+    bool shallow = false;
 
     GitInput(const ParsedURL & url) : url(url)
     { }
@@ -114,6 +115,7 @@ struct GitInput : Input
         if (url2.scheme != "git") url2.scheme = "git+" + url2.scheme;
         if (rev) url2.query.insert_or_assign("rev", rev->gitRev());
         if (ref) url2.query.insert_or_assign("ref", *ref);
+        if (shallow) url2.query.insert_or_assign("shallow", "1");
         return url2.to_string();
     }
 
@@ -125,6 +127,8 @@ struct GitInput : Input
             attrs.emplace("ref", *ref);
         if (rev)
             attrs.emplace("rev", rev->gitRev());
+        if (shallow)
+            attrs.emplace("shallow", true);
         return attrs;
     }
 
@@ -361,9 +365,12 @@ struct GitInput : Input
 
         bool isShallow = chomp(runProgram("git", true, { "-C", repoDir, "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);
+
         if (auto tree = lookupGitInfo(store, name, *input->rev)) {
             assert(*input->rev == tree->first);
-            if (isShallow) tree->second.info.revCount.reset();
+            if (shallow) tree->second.info.revCount.reset();
             return {std::move(tree->second), input};
         }
 
@@ -393,7 +400,7 @@ struct GitInput : Input
             .storePath = std::move(storePath),
             .info = TreeInfo {
                 .revCount =
-                    !isShallow
+                    !shallow
                     ? std::optional(std::stoull(runProgram("git", true, { "-C", repoDir, "rev-list", "--count", input->rev->gitRev() })))
                     : std::nullopt,
                 .lastModified = lastModified
@@ -440,7 +447,7 @@ struct GitInputScheme : InputScheme
         if (maybeGetStrAttr(attrs, "type") != "git") return {};
 
         for (auto & [name, value] : attrs)
-            if (name != "type" && name != "url" && name != "ref" && name != "rev")
+            if (name != "type" && name != "url" && name != "ref" && name != "rev" && name != "shallow")
                 throw Error("unsupported Git input attribute '%s'", name);
 
         auto input = std::make_unique<GitInput>(parseURL(getStrAttr(attrs, "url")));
@@ -451,6 +458,9 @@ struct GitInputScheme : InputScheme
         }
         if (auto rev = maybeGetStrAttr(attrs, "rev"))
             input->rev = Hash(*rev, htSHA1);
+
+        input->shallow = maybeGetBoolAttr(attrs, "shallow").value_or(false);
+
         return input;
     }
 };
diff --git a/src/libstore/fetchers/mercurial.cc b/src/libstore/fetchers/mercurial.cc
index a9c86f1b4..3a767ef17 100644
--- a/src/libstore/fetchers/mercurial.cc
+++ b/src/libstore/fetchers/mercurial.cc
@@ -260,7 +260,7 @@ struct MercurialInput : Input
 
         Attrs infoAttrs({
             {"rev", input->rev->gitRev()},
-            {"revCount", revCount},
+            {"revCount", (int64_t) revCount},
         });
 
         if (!this->rev)
diff --git a/tests/fetchGit.sh b/tests/fetchGit.sh
index 6e5a4750e..bca581438 100644
--- a/tests/fetchGit.sh
+++ b/tests/fetchGit.sh
@@ -148,8 +148,12 @@ NIX=$(command -v nix)
 path5=$(nix eval --impure --raw --expr "(builtins.fetchGit { url = $repo; ref = \"dev\"; }).outPath")
 [[ $path3 = $path5 ]]
 
-# Check that shallow clones work and don't return a revcount.
+# Fetching a shallow repo shouldn't work by default, because we can't
+# return a revCount.
 git clone --depth 1 file://$repo $TEST_ROOT/shallow
-path6=$(nix eval --impure --raw --expr "(builtins.fetchGit { url = $TEST_ROOT/shallow; ref = \"dev\"; }).outPath")
+(! nix eval --impure --raw --expr "(builtins.fetchGit { url = $TEST_ROOT/shallow; ref = \"dev\"; }).outPath")
+
+# But you can request a shallow clone, which won't return a revCount.
+path6=$(nix eval --impure --raw --expr "(builtins.fetchTree { type = \"git\"; url = \"file://$TEST_ROOT/shallow\"; ref = \"dev\"; shallow = true; }).outPath")
 [[ $path3 = $path6 ]]
-[[ $(nix eval --impure --expr "(builtins.fetchTree { type = \"git\"; url = \"file://$TEST_ROOT/shallow\"; ref = \"dev\"; }).revCount or 123") == 123 ]]
+[[ $(nix eval --impure --expr "(builtins.fetchTree { type = \"git\"; url = \"file://$TEST_ROOT/shallow\"; ref = \"dev\"; shallow = true; }).revCount or 123") == 123 ]]