Don't track un-prefixed namespaces

2 files changed, 24 insertions, 6 deletions

diff --git a/bs4/builder/_lxml.py b/bs4/builder/_lxml.py
index 3a1700c..b7e172c 100644
--- a/bs4/builder/_lxml.py
+++ b/bs4/builder/_lxml.py
@@ -71,12 +71,12 @@ class LXMLTreeBuilderForXML(TreeBuilder):
         This might be useful later on when creating CSS selectors.
         """
         for key, value in mapping.items():
-            if key not in self.soup._namespaces:
-                    # Let the BeautifulSoup object know about a new namespace.
-                    # If there are multiple namespaces defined with the same
-                    # prefix, the first one in the document takes precedence.
-                    self.soup._namespaces[key] = value
-        
+            if key and key not in self.soup._namespaces:
+                # Let the BeautifulSoup object know about a new namespace.
+                # If there are multiple namespaces defined with the same
+                # prefix, the first one in the document takes precedence.
+                self.soup._namespaces[key] = value
+
     def default_parser(self, encoding):
         # This can either return a parser object or a class, which
         # will be instantiated with default arguments.
diff --git a/bs4/tests/test_lxml.py b/bs4/tests/test_lxml.py
index 8a8f690..1a4f27c 100644
--- a/bs4/tests/test_lxml.py
+++ b/bs4/tests/test_lxml.py
@@ -80,3 +80,21 @@ class LXMLXMLTreeBuilderSmokeTest(SoupTest, XMLTreeBuilderSmokeTest):
     @property
     def default_builder(self):
         return LXMLTreeBuilderForXML()
+
+    def test_namespace_indexing(self):
+        # We should not track un-prefixed namespaces as we can only hold one
+        # and it will be recognized as the default namespace by soupsieve,
+        # which may be confusing in some situations. When no namespace is provided
+        # for a selector, the default namespace (if defined) is assumed.
+
+        soup = self.soup(
+            '<?xml version="1.1"?>\n'
+            '<root>'
+            '<tag xmlns="http://unprefixed-namespace.com">content</tag>'
+            '<prefix:tag xmlns:prefix="http://prefixed-namespace.com">content</tag>'
+            '</root>'
+        )
+        self.assertEqual(
+            soup._namespaces,
+            {'xml': 'http://www.w3.org/XML/1998/namespace', 'prefix': 'http://prefixed-namespace.com'}
+        )