diff --git a/sphinx/environment/collectors/metadata.py b/sphinx/environment/collectors/metadata.py
index fdc3627bc..c55b8458e 100644
--- a/sphinx/environment/collectors/metadata.py
+++ b/sphinx/environment/collectors/metadata.py
@@ -37,44 +37,37 @@ class MetadataCollector(EnvironmentCollector):
             env.metadata[docname] = other.metadata[docname]
 
     def process_doc(self, app, doctree):
-        # type: (Sphinx, nodes.Node) -> None
+        # type: (Sphinx, nodes.document) -> None
         """Process the docinfo part of the doctree as metadata.
 
         Keep processing minimal -- just return what docutils says.
         """
-        md = app.env.metadata[app.env.docname]
-        try:
-            docinfo = doctree[0]
-        except IndexError:
-            # probably an empty document
-            return
-        if docinfo.__class__ is not nodes.docinfo:
-            # nothing to see here
-            return
-        for node in docinfo:
-            # nodes are multiply inherited...
-            if isinstance(node, nodes.authors):
-                authors = cast(List[nodes.author], node)
-                md['authors'] = [author.astext() for author in authors]
-            elif isinstance(node, nodes.field):
-                assert len(node) == 2
-                field_name = cast(nodes.field_name, node[0])
-                field_body = cast(nodes.field_body, node[1])
-                md[field_name.astext()] = field_body.astext()
-            else:
-                # other children must be TextElement
-                # see: http://docutils.sourceforge.net/docs/ref/doctree.html#bibliographic-elements  # NOQA
-                element = cast(nodes.TextElement, node)
-                md[element.__class__.__name__] = element.astext()
-        for name, value in md.items():
-            if name in ('tocdepth',):
-                try:
-                    value = int(value)
-                except ValueError:
-                    value = 0
-                md[name] = value
+        if len(doctree) > 0 and isinstance(doctree[0], nodes.docinfo):
+            md = app.env.metadata[app.env.docname]
+            for node in doctree[0]:
+                # nodes are multiply inherited...
+                if isinstance(node, nodes.authors):
+                    authors = cast(List[nodes.author], node)
+                    md['authors'] = [author.astext() for author in authors]
+                elif isinstance(node, nodes.field):
+                    assert len(node) == 2
+                    field_name = cast(nodes.field_name, node[0])
+                    field_body = cast(nodes.field_body, node[1])
+                    md[field_name.astext()] = field_body.astext()
+                elif isinstance(node, nodes.TextElement):
+                    # other children must be TextElement
+                    # see: http://docutils.sourceforge.net/docs/ref/doctree.html#bibliographic-elements  # NOQA
+                    md[node.__class__.__name__] = node.astext()
 
-        del doctree[0]
+            for name, value in md.items():
+                if name in ('tocdepth',):
+                    try:
+                        value = int(value)
+                    except ValueError:
+                        value = 0
+                    md[name] = value
+
+            doctree.pop(0)
 
 
 def setup(app):