diff --git a/impl/maven-xml/src/main/java/org/apache/maven/internal/xml/XmlNodeImpl.java b/impl/maven-xml/src/main/java/org/apache/maven/internal/xml/XmlNodeImpl.java
index f553b87f508e..4d49b20e5d11 100644
--- a/impl/maven-xml/src/main/java/org/apache/maven/internal/xml/XmlNodeImpl.java
+++ b/impl/maven-xml/src/main/java/org/apache/maven/internal/xml/XmlNodeImpl.java
@@ -224,7 +224,7 @@ public static XmlNode merge(XmlNode dominant, XmlNode recessive, Boolean childMe
 
         boolean mergeSelf = true;
 
-        String selfMergeMode = dominant.getAttribute(SELF_COMBINATION_MODE_ATTRIBUTE);
+        String selfMergeMode = getSelfCombinationMode(dominant);
 
         if (SELF_COMBINATION_OVERRIDE.equals(selfMergeMode)) {
             mergeSelf = false;
@@ -252,8 +252,8 @@ public static XmlNode merge(XmlNode dominant, XmlNode recessive, Boolean childMe
                 if (childMergeOverride != null) {
                     mergeChildren = childMergeOverride;
                 } else {
-                    String childMergeMode = attrs.get(CHILDREN_COMBINATION_MODE_ATTRIBUTE);
-                    if (CHILDREN_COMBINATION_APPEND.equals(childMergeMode)) {
+                    String childCombinationMode = getChildCombinationMode(attrs);
+                    if (CHILDREN_COMBINATION_APPEND.equals(childCombinationMode)) {
                         mergeChildren = false;
                     }
                 }
@@ -324,8 +324,7 @@ public static XmlNode merge(XmlNode dominant, XmlNode recessive, Boolean childMe
                         } else if (it.hasNext()) {
                             XmlNode dominantChild = it.next();
 
-                            String dominantChildCombinationMode =
-                                    dominantChild.getAttribute(SELF_COMBINATION_MODE_ATTRIBUTE);
+                            String dominantChildCombinationMode = getSelfCombinationMode(dominantChild);
                             if (SELF_COMBINATION_REMOVE.equals(dominantChildCombinationMode)) {
                                 if (children == null) {
                                     children = new ArrayList<>(dominant.getChildren());
@@ -371,6 +370,36 @@ public static XmlNode merge(XmlNode dominant, XmlNode recessive, Boolean childMe
         return dominant;
     }
 
+    private static String getChildCombinationMode(Map<String, String> attrs) {
+        String attribute = attrs.get(CHILDREN_COMBINATION_MODE_ATTRIBUTE);
+        if (attribute == null) {
+            return DEFAULT_CHILDREN_COMBINATION_MODE;
+        }
+        if (CHILDREN_COMBINATION_APPEND.equals(attribute) || CHILDREN_COMBINATION_MERGE.equals(attribute)) {
+            return attribute;
+        }
+        throw new IllegalArgumentException("Unsupported value '" + attribute + "' for "
+                + CHILDREN_COMBINATION_MODE_ATTRIBUTE + " attribute. " + "Valid values are: "
+                + CHILDREN_COMBINATION_APPEND + ", and " + CHILDREN_COMBINATION_MERGE
+                + " (default is: " + DEFAULT_SELF_COMBINATION_MODE + ")");
+    }
+
+    private static String getSelfCombinationMode(XmlNode node) {
+        String attribute = node.getAttribute(SELF_COMBINATION_MODE_ATTRIBUTE);
+        if (attribute == null) {
+            return DEFAULT_SELF_COMBINATION_MODE;
+        }
+        if (SELF_COMBINATION_OVERRIDE.equals(attribute)
+                || SELF_COMBINATION_MERGE.equals(attribute)
+                || SELF_COMBINATION_REMOVE.equals(attribute)) {
+            return attribute;
+        }
+        throw new IllegalArgumentException("Unsupported value '" + attribute + "' for "
+                + SELF_COMBINATION_MODE_ATTRIBUTE + " attribute. " + "Valid values are: "
+                + SELF_COMBINATION_OVERRIDE + ", " + SELF_COMBINATION_MERGE + ", and " + SELF_COMBINATION_REMOVE
+                + " (default is: " + DEFAULT_SELF_COMBINATION_MODE + ")");
+    }
+
     /**
      * Merge two DOMs, with one having dominance in the case of collision. Merge mechanisms (vs. override for nodes, or
      * vs. append for children) is determined by attributes of the dominant root node.