diff --git a/prism/src/parser/ast/ASTElement.java b/prism/src/parser/ast/ASTElement.java
index fdb387de..2ecfd45d 100644
--- a/prism/src/parser/ast/ASTElement.java
+++ b/prism/src/parser/ast/ASTElement.java
@@ -271,7 +271,9 @@ public abstract class ASTElement
 
 	/**
 	 * Get all undefined constants used (i.e. in ExpressionConstant objects) recursively and return as a list.
-	 * Recursive decent means that we find e.g. constants that are used within other constants, labels.
+	 * Recursive descent means that we find e.g. constants that are used within other constants, labels.
+	 * But note that we only look at/for constants in the passed in ConstantList.
+	 * Any others discovered are ignored (and not descended into).
 	 */
 	public Vector<String> getAllUndefinedConstantsRecursively(ConstantList constantList, LabelList labelList)
 	{
diff --git a/prism/src/parser/ast/PropertiesFile.java b/prism/src/parser/ast/PropertiesFile.java
index ebd36208..9d36d703 100644
--- a/prism/src/parser/ast/PropertiesFile.java
+++ b/prism/src/parser/ast/PropertiesFile.java
@@ -366,7 +366,7 @@ public class PropertiesFile extends ASTElement
 	
 	/**
 	 * Get a list of undefined (properties file) constants appearing in labels of the properties file
-	 * (undefined constants are those of form "const int x;" rather than "const int x = 1;") 
+	 * (undefined constants are those of form "const int x;" rather than "const int x = 1;")
 	 */
 	public Vector<String> getUndefinedConstantsUsedInLabels()
 	{
diff --git a/prism/src/parser/visitor/GetAllUndefinedConstantsRecursively.java b/prism/src/parser/visitor/GetAllUndefinedConstantsRecursively.java
index 329f599e..b76ad0cc 100644
--- a/prism/src/parser/visitor/GetAllUndefinedConstantsRecursively.java
+++ b/prism/src/parser/visitor/GetAllUndefinedConstantsRecursively.java
@@ -33,7 +33,9 @@ import prism.PrismLangException;
 
 /**
  * Get all undefined constants used (i.e. in ExpressionConstant objects) recursively and return as a list.
- * Recursive decent means that we find e.g. constants that are used within other constants, labels.
+ * Recursive descent means that we find e.g. constants that are used within other constants, labels.
+ * But note that we only look at/for constants in the passed in ConstantList.
+ * Any others discovered are ignored (and not descended into).
  */
 public class GetAllUndefinedConstantsRecursively extends ASTTraverse
 {
@@ -52,8 +54,9 @@ public class GetAllUndefinedConstantsRecursively extends ASTTraverse
 	{
 		// Look up this constant in the constant list
 		int i = constantList.getConstantIndex(e.getName());
+		// Ignore constants not in the list 
 		if (i == -1)
-			throw new PrismLangException("Unknown constant \"" + e.getName() + "\"");
+			return;
 		Expression expr = constantList.getConstant(i);
 		// If constant is undefined, add to the list
 		if (expr == null) {