diff --git a/lib/metanorma/standoc/validate.rb b/lib/metanorma/standoc/validate.rb
index 6a44b88b..3ab85005 100644
--- a/lib/metanorma/standoc/validate.rb
+++ b/lib/metanorma/standoc/validate.rb
@@ -219,7 +219,7 @@ def repeat_id_validate(doc)
 
       # manually check for xref/@target, xref/@to integrity
       def xref_validate(doc)
-        @doc_xrefs = doc.xpath("//xref/@target | //xref/@to")
+        @doc_xrefs = doc.xpath("//xref/@target | //xref/@to | //index/@to")
           .each_with_object({}) do |x, m|
           m[x.text] = x
           @doc_ids[x.text] and next
diff --git a/spec/metanorma/validate_spec.rb b/spec/metanorma/validate_spec.rb
index 333d6d1d..30c2ca6e 100644
--- a/spec/metanorma/validate_spec.rb
+++ b/spec/metanorma/validate_spec.rb
@@ -1266,7 +1266,7 @@
       .to include("Could not resolve footnoteblock:[id1]")
   end
 
-  it "Warning if xref/@target does not point to a real identifier" do
+  it "Warning if xref/@target, (xref/@to), index/@to does not point to a real identifier" do
     FileUtils.rm_f "test.err.html"
     Asciidoctor.convert(<<~INPUT, *OPTIONS)
       = Document title
@@ -1275,9 +1275,12 @@
       :no-pdf:
 
       <<id1>>
+      index-range:id3[(((A)))]
     INPUT
     expect(File.read("test.err.html"))
       .to include("Crossreference target id1 is undefined")
+    expect(File.read("test.err.html"))
+      .to include("Crossreference target id3 is undefined")
   end
 
   it "Warns if illegal nessting of assets within assets" do