diff --git a/README.md b/README.md
index 9435106b2..d0e5545a7 100644
--- a/README.md
+++ b/README.md
@@ -39,6 +39,7 @@ This project was the following modules:
     * Added new methods `EmailAddress.createOnDemand`
     * `URLHelper.urlEncode` and `URLHelper.urlDecode` now uses `URLCodec`
     * Deprecated `ICommonsIterable.forEach` in favour of `findAll`
+    * Fixed a bug in the cloning of `MapBasedNamespaceContext` (see [issue #17](https://github.com/phax/ph-commons/issues/17))
 * v9.3.9 - 2019-12-11
     * Made `ClassLoaderHelper.getResource` more robust
     * Updated "mime-type-info.xml" list with  shared-mime-info-spec 1.15
diff --git a/ph-xml/src/main/java/com/helger/xml/namespace/MapBasedNamespaceContext.java b/ph-xml/src/main/java/com/helger/xml/namespace/MapBasedNamespaceContext.java
index 1fc7fb696..a5acddead 100644
--- a/ph-xml/src/main/java/com/helger/xml/namespace/MapBasedNamespaceContext.java
+++ b/ph-xml/src/main/java/com/helger/xml/namespace/MapBasedNamespaceContext.java
@@ -70,7 +70,10 @@ public MapBasedNamespaceContext (@Nullable final MapBasedNamespaceContext aOther
     {
       m_sDefaultNamespaceURI = aOther.m_sDefaultNamespaceURI;
       m_aPrefix2NS.putAll (aOther.m_aPrefix2NS);
-      m_aNS2Prefix.putAll (aOther.m_aNS2Prefix);
+
+      // putAll is not enough here
+      for (final Map.Entry <String, ICommonsSet <String>> aEntry : aOther.m_aNS2Prefix.entrySet ())
+        m_aNS2Prefix.put (aEntry.getKey (), aEntry.getValue ().getClone ());
     }
   }
 
diff --git a/ph-xml/src/test/java/com/helger/xml/namespace/MapBasedNamespaceContextTest.java b/ph-xml/src/test/java/com/helger/xml/namespace/MapBasedNamespaceContextTest.java
index 2d717ea53..fc2646af0 100644
--- a/ph-xml/src/test/java/com/helger/xml/namespace/MapBasedNamespaceContextTest.java
+++ b/ph-xml/src/test/java/com/helger/xml/namespace/MapBasedNamespaceContextTest.java
@@ -20,6 +20,7 @@
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
 import static org.junit.Assert.assertTrue;
 
 import javax.xml.XMLConstants;
@@ -27,6 +28,7 @@
 import org.junit.Test;
 
 import com.helger.commons.mock.CommonsTestHelper;
+import com.helger.commons.state.EChange;
 
 /**
  * Test class for class {@link MapBasedNamespaceContext}.
@@ -91,4 +93,19 @@ public void testAll ()
     CommonsTestHelper.testToStringImplementation (c);
     CommonsTestHelper.testDefaultSerialization (c);
   }
+
+  @Test
+  public void testCloneIssue17 ()
+  {
+    final MapBasedNamespaceContext aNSCtx = new MapBasedNamespaceContext ();
+    aNSCtx.addMapping ("p1", "urn:example1");
+    aNSCtx.addMapping ("p2", "urn:example2");
+    final MapBasedNamespaceContext aNSCtx2 = aNSCtx.getClone ();
+    // Remove from original
+    assertSame (EChange.CHANGED, aNSCtx.removeMapping ("p1"));
+    assertSame (EChange.UNCHANGED, aNSCtx.removeMapping ("p1"));
+    // Remove from clone
+    assertSame (EChange.CHANGED, aNSCtx2.removeMapping ("p1"));
+    assertSame (EChange.UNCHANGED, aNSCtx2.removeMapping ("p1"));
+  }
 }