diff --git a/classlib/src/main/java/org/teavm/classlib/java/util/TProperties.java b/classlib/src/main/java/org/teavm/classlib/java/util/TProperties.java
index c80950b57..a8f37d9fa 100644
--- a/classlib/src/main/java/org/teavm/classlib/java/util/TProperties.java
+++ b/classlib/src/main/java/org/teavm/classlib/java/util/TProperties.java
@@ -331,7 +331,7 @@ public class TProperties extends THashtable<Object, Object> {
         return selected.keys();
     }
 
-    private void selectProperties(THashtable<Object, Object> selected) {
+    private void selectProperties(TMap<Object, Object> selected) {
         if (defaults != null) {
             defaults.selectProperties(selected);
         }
@@ -398,4 +398,24 @@ public class TProperties extends THashtable<Object, Object> {
         }
         writer.write("\n");
     }
+
+    public TSet<String> stringPropertyNames() {
+        TSet<String> selected = new THashSet<>();
+        selectPropertyNames(selected);
+        return selected;
+    }
+
+    private void selectPropertyNames(TSet<String> selected) {
+        if (defaults != null) {
+            defaults.selectPropertyNames(selected);
+        }
+
+        TEnumeration<Object> e = keys();
+        while (e.hasMoreElements()) {
+            Object key = e.nextElement();
+            if (key instanceof String) {
+                selected.add((String) key);
+            }
+        }
+    }
 }
diff --git a/core/src/main/java/org/teavm/parsing/ClasspathResourceMapper.java b/core/src/main/java/org/teavm/parsing/ClasspathResourceMapper.java
index 42513dfeb..3b5955a30 100644
--- a/core/src/main/java/org/teavm/parsing/ClasspathResourceMapper.java
+++ b/core/src/main/java/org/teavm/parsing/ClasspathResourceMapper.java
@@ -67,6 +67,7 @@ public class ClasspathResourceMapper implements Mapper<String, ClassHolder>, Cla
         Map<String, Transformation> transformationMap = new HashMap<>();
         loadProperties(properties, transformationMap);
         transformations.addAll(transformationMap.values());
+        renamer = new ClassRefsRenamer(new CachedMapper<>(classNameMapper));
     }
 
     private void loadProperties(Properties properties, Map<String, Transformation> cache) {
@@ -165,6 +166,9 @@ public class ClasspathResourceMapper implements Mapper<String, ClassHolder>, Cla
     }
 
     private Date getOriginalModificationDate(String className) {
+        if (classLoader == null) {
+            return null;
+        }
         URL url = classLoader.getResource(className.replace('.', '/') + ".class");
         if (url == null) {
             return null;