diff --git a/CHANGES b/CHANGES
index 57234ee3a..f8d199e6f 100644
--- a/CHANGES
+++ b/CHANGES
@@ -28,6 +28,8 @@ Bugs fixed
 * #5391: A cross reference in heading is rendered as literal
 * #5946: C++, fix ``cpp:alias`` problems in LaTeX (and singlehtml)
 * #6147: classes attribute of ``citation_reference`` node is lost
+* AssertionError is raised when custom ``citation_reference`` node having
+  classes attribute refers missing citation (refs: #6147)
 
 Testing
 --------
diff --git a/sphinx/transforms/__init__.py b/sphinx/transforms/__init__.py
index 9ff5aeccd..6f513377b 100644
--- a/sphinx/transforms/__init__.py
+++ b/sphinx/transforms/__init__.py
@@ -220,7 +220,7 @@ class CitationReferences(SphinxTransform):
                                             ids=citation_ref["ids"])
             refnode.source = citation_ref.source or citation_ref.parent.source
             refnode.line = citation_ref.line or citation_ref.parent.line
-            refnode += nodes.Text('[' + cittext + ']')
+            refnode += nodes.inline(cittext, '[%s]' % cittext)
             for class_name in citation_ref.attributes.get('classes', []):
                 refnode['classes'].append(class_name)
             citation_ref.parent.replace(citation_ref, refnode)
diff --git a/tests/test_build_html.py b/tests/test_build_html.py
index 99d7bf8d4..1dbf05a4a 100644
--- a/tests/test_build_html.py
+++ b/tests/test_build_html.py
@@ -391,8 +391,8 @@ def test_html4_output(app, status, warning):
         (".//a[@class='footnote-reference brackets'][@href='#id9'][@id='id1']", r"1"),
         (".//a[@class='footnote-reference brackets'][@href='#id10'][@id='id2']", r"2"),
         (".//a[@class='footnote-reference brackets'][@href='#foo'][@id='id3']", r"3"),
-        (".//a[@class='reference internal'][@href='#bar'][@id='id4']", r"\[bar\]"),
-        (".//a[@class='reference internal'][@href='#baz-qux'][@id='id5']", r"\[baz_qux\]"),
+        (".//a[@class='reference internal'][@href='#bar'][@id='id4']/span", r"\[bar\]"),
+        (".//a[@class='reference internal'][@href='#baz-qux'][@id='id5']/span", r"\[baz_qux\]"),
         (".//a[@class='footnote-reference brackets'][@href='#id11'][@id='id6']", r"4"),
         (".//a[@class='footnote-reference brackets'][@href='#id12'][@id='id7']", r"5"),
         (".//a[@class='fn-backref'][@href='#id1']", r"1"),