diff --git a/src/libexpr/eval-cache.cc b/src/libexpr/eval-cache.cc
index 75e9af787..98d91c905 100644
--- a/src/libexpr/eval-cache.cc
+++ b/src/libexpr/eval-cache.cc
@@ -394,7 +394,7 @@ Value & AttrCursor::forceValue()
             cachedValue = {root->db->setString(getKey(), v.string.s, v.string.context),
                            string_t{v.string.s, {}}};
         else if (v.type() == nPath)
-            cachedValue = {root->db->setString(getKey(), v.path), v.path};
+            cachedValue = {root->db->setString(getKey(), v.path), string_t{v.path, {}}};
         else if (v.type() == nBool)
             cachedValue = {root->db->setBool(getKey(), v.boolean), v.boolean};
         else if (v.type() == nAttrs)
diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index 133299030..ab80be2d3 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -104,7 +104,7 @@ static void fetchTree(
             else if (attr.value->type() == nBool)
                 attrs.emplace(attr.name, Explicit<bool>{attr.value->boolean});
             else if (attr.value->type() == nInt)
-                attrs.emplace(attr.name, attr.value->integer);
+                attrs.emplace(attr.name, uint64_t(attr.value->integer));
             else
                 throw TypeError("fetchTree argument '%s' is %s while a string, Boolean or integer is expected",
                     attr.name, showType(*attr.value));
diff --git a/src/libfetchers/attrs.cc b/src/libfetchers/attrs.cc
index 720b19fcd..17fc4041f 100644
--- a/src/libfetchers/attrs.cc
+++ b/src/libfetchers/attrs.cc
@@ -11,7 +11,7 @@ Attrs jsonToAttrs(const nlohmann::json & json)
 
     for (auto & i : json.items()) {
         if (i.value().is_number())
-            attrs.emplace(i.key(), i.value().get<int64_t>());
+            attrs.emplace(i.key(), i.value().get<uint64_t>());
         else if (i.value().is_string())
             attrs.emplace(i.key(), i.value().get<std::string>());
         else if (i.value().is_boolean())
diff --git a/src/libfetchers/github.cc b/src/libfetchers/github.cc
index 92ff224f7..db1ced5d6 100644
--- a/src/libfetchers/github.cc
+++ b/src/libfetchers/github.cc
@@ -195,14 +195,14 @@ struct GitArchiveInputScheme : InputScheme
 
         auto [tree, lastModified] = downloadTarball(store, url.url, "source", true, url.headers);
 
-        input.attrs.insert_or_assign("lastModified", lastModified);
+        input.attrs.insert_or_assign("lastModified", uint64_t(lastModified));
 
         getCache()->add(
             store,
             immutableAttrs,
             {
                 {"rev", rev->gitRev()},
-                {"lastModified", lastModified}
+                {"lastModified", uint64_t(lastModified)}
             },
             tree.storePath,
             true);
diff --git a/src/libfetchers/mercurial.cc b/src/libfetchers/mercurial.cc
index 07a51059d..0eb401e10 100644
--- a/src/libfetchers/mercurial.cc
+++ b/src/libfetchers/mercurial.cc
@@ -301,7 +301,7 @@ struct MercurialInputScheme : InputScheme
 
         Attrs infoAttrs({
             {"rev", input.getRev()->gitRev()},
-            {"revCount", (int64_t) revCount},
+            {"revCount", (uint64_t) revCount},
         });
 
         if (!_input.getRev())
diff --git a/src/libfetchers/tarball.cc b/src/libfetchers/tarball.cc
index 8c0f20475..56c014a8c 100644
--- a/src/libfetchers/tarball.cc
+++ b/src/libfetchers/tarball.cc
@@ -152,7 +152,7 @@ std::pair<Tree, time_t> downloadTarball(
     }
 
     Attrs infoAttrs({
-        {"lastModified", lastModified},
+        {"lastModified", uint64_t(lastModified)},
         {"etag", res.etag},
     });