diff --git a/pmd/src/net/sourceforge/pmd/ExternalRuleID.java b/pmd/src/net/sourceforge/pmd/ExternalRuleID.java
index a46919a8c8..74cb409660 100644
--- a/pmd/src/net/sourceforge/pmd/ExternalRuleID.java
+++ b/pmd/src/net/sourceforge/pmd/ExternalRuleID.java
@@ -12,6 +12,9 @@ public class ExternalRuleID {
 
     public ExternalRuleID(String id) {
         int afterXML = id.indexOf(".xml") + 5;
+        if (afterXML >= id.length()) {
+            throw new RuntimeException("Unable to parse reference to external rule " + id +".  These references need to be in the form <rulesetname>/<rulename>, i.e., <rule ref=\"rulesets/unusedcode.xml/UnusedPrivateField\"/>");
+        }
         filename = id.substring(0, afterXML-1);
         ruleName = id.substring(afterXML);
     }
diff --git a/pmd/src/net/sourceforge/pmd/RuleSetFactory.java b/pmd/src/net/sourceforge/pmd/RuleSetFactory.java
index 6dc24c2b70..611a3476cf 100644
--- a/pmd/src/net/sourceforge/pmd/RuleSetFactory.java
+++ b/pmd/src/net/sourceforge/pmd/RuleSetFactory.java
@@ -40,6 +40,10 @@ public class RuleSetFactory {
         }
     }
 
+    /**
+     * Creates a ruleset.  If passed a comma-delimited string (rulesets/basic.xml,rulesets/unusedcode.xml)
+     * it will parse that string and create a new ruleset for each item in the list.
+     */
     public RuleSet createRuleSet(String name) throws RuleSetNotFoundException {
         if (name.indexOf(',') == -1) {
            return createRuleSet(tryToGetStreamTo(name));