diff --git a/impl/maven-impl/src/main/java/org/apache/maven/impl/model/DefaultInheritanceAssembler.java b/impl/maven-impl/src/main/java/org/apache/maven/impl/model/DefaultInheritanceAssembler.java
index aeacef7b530a..b6d7230b4f41 100644
--- a/impl/maven-impl/src/main/java/org/apache/maven/impl/model/DefaultInheritanceAssembler.java
+++ b/impl/maven-impl/src/main/java/org/apache/maven/impl/model/DefaultInheritanceAssembler.java
@@ -105,7 +105,8 @@ private String getChildPathAdjustment(Model child, Model parent, String childDir
              * repository. In other words, modules where artifactId != moduleDirName will see different effective URLs
              * depending on how the model was constructed (from filesystem or from repository).
              */
-            if (child.getProjectDirectory() != null) {
+            if (child.getProjectDirectory() != null
+                    && child.getProjectDirectory().getFileName() != null) {
                 childName = child.getProjectDirectory().getFileName().toString();
             }
 
diff --git a/impl/maven-impl/src/test/java/org/apache/maven/impl/model/DefaultInheritanceAssemblerTest.java b/impl/maven-impl/src/test/java/org/apache/maven/impl/model/DefaultInheritanceAssemblerTest.java
index e8ee2d37a111..b7e32d3c89ff 100644
--- a/impl/maven-impl/src/test/java/org/apache/maven/impl/model/DefaultInheritanceAssemblerTest.java
+++ b/impl/maven-impl/src/test/java/org/apache/maven/impl/model/DefaultInheritanceAssemblerTest.java
@@ -21,6 +21,7 @@
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.Paths;
+import java.util.List;
 
 import org.apache.maven.api.model.Model;
 import org.apache.maven.api.services.xml.XmlReaderRequest;
@@ -31,6 +32,7 @@
 import org.xmlunit.builder.DiffBuilder;
 import org.xmlunit.diff.Diff;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 
 class DefaultInheritanceAssemblerTest {
@@ -58,6 +60,76 @@ void testPluginConfiguration() throws Exception {
         testInheritance("plugin-configuration");
     }
 
+    /**
+     * Check most classical urls inheritance: directory structure where parent POM in parent directory
+     * and child directory == artifactId
+     */
+    @Test
+    void testUrls() throws Exception {
+        testInheritance("urls");
+    }
+
+    /**
+     * Flat directory structure: parent & child POMs in sibling directories, child directory == artifactId.
+     */
+    @Test
+    void testFlatUrls() throws Exception {
+        testInheritance("flat-urls");
+    }
+
+    /**
+     * MNG-5951 MNG-6059 child.x.y.inherit.append.path="false" test
+     */
+    @Test
+    void testNoAppendUrls() throws Exception {
+        testInheritance("no-append-urls");
+    }
+
+    /**
+     * MNG-5951 special case test: inherit with partial override
+     */
+    @Test
+    void testNoAppendUrls2() throws Exception {
+        testInheritance("no-append-urls2");
+    }
+
+    /**
+     * MNG-5951 special case test: child.x.y.inherit.append.path="true" in child should not reset content
+     */
+    @Test
+    void testNoAppendUrls3() throws Exception {
+        testInheritance("no-append-urls3");
+    }
+
+    @Test
+    void testWithEmptyUrl() throws Exception {
+        testInheritance("empty-urls", false);
+    }
+
+    @Test
+    void testModulePathNotArtifactId() throws Exception {
+        Model parent = getModel("module-path-not-artifactId-parent");
+        Model child = getModel("module-path-not-artifactId-child");
+
+        Model assembled = assembler.assembleModelInheritance(child, parent, null, null);
+
+        Path actual = Paths.get("target/test-classes/poms/inheritance/module-path-not-artifactId-actual.xml");
+        Files.createDirectories(actual.getParent());
+        xmlFactory.write(XmlWriterRequest.<Model>builder()
+                .content(assembled)
+                .path(actual)
+                .build());
+
+        Path expected = getPom("module-path-not-artifactId-expected");
+
+        Diff diff = DiffBuilder.compare(expected.toFile())
+                .withTest(actual.toFile())
+                .ignoreComments()
+                .ignoreWhitespace()
+                .build();
+        assertFalse(diff.hasDifferences(), "XML files should be identical: " + diff.toString());
+    }
+
     public void testInheritance(String baseName) throws Exception {
         testInheritance(baseName, false);
         testInheritance(baseName, true);
@@ -95,4 +167,62 @@ public void testInheritance(String baseName, boolean fromRepo) throws Exception
                 .build();
         assertFalse(diff.hasDifferences(), "XML files should be identical: " + diff.toString());
     }
+
+    @Test
+    void testAssembleWithNullArtifactIdDoesNotThrowNpe() {
+        Model parent = Model.newBuilder()
+                .modelVersion("4.0.0")
+                .groupId("test")
+                .artifactId("parent")
+                .version("1.0")
+                .build();
+
+        Model child = Model.newBuilder()
+                .modelVersion("4.0.0")
+                .groupId("test")
+                .version("1.0")
+                .build();
+
+        assertDoesNotThrow(() -> assembler.assembleModelInheritance(child, parent, null, null));
+    }
+
+    @Test
+    void testAssembleWithRootProjectDirectoryDoesNotThrowNpe() {
+        Model parent = Model.newBuilder()
+                .modelVersion("4.0.0")
+                .groupId("test")
+                .artifactId("parent")
+                .version("1.0")
+                .build();
+
+        Model child = Model.newBuilder()
+                .modelVersion("4.0.0")
+                .groupId("test")
+                .artifactId("child")
+                .version("1.0")
+                .pomFile(Paths.get("/pom.xml"))
+                .build();
+
+        assertDoesNotThrow(() -> assembler.assembleModelInheritance(child, parent, null, null));
+    }
+
+    @Test
+    void testAssembleWithNullArtifactIdAndRootProjectDirectoryDoesNotThrowNpe() {
+        Model parent = Model.newBuilder()
+                .modelVersion("4.0.0")
+                .groupId("test")
+                .artifactId("parent")
+                .version("1.0")
+                .modules(List.of("../child/pom.xml"))
+                .build();
+
+        Model child = Model.newBuilder()
+                .modelVersion("4.0.0")
+                .groupId("test")
+                .version("1.0")
+                .pomFile(Paths.get("/pom.xml"))
+                .build();
+
+        assertDoesNotThrow(() -> assembler.assembleModelInheritance(child, parent, null, null));
+    }
 }