diff --git src/java/org/apache/nutch/util/URLUtil.java src/java/org/apache/nutch/util/URLUtil.java
index 1e50b8e..dae2ad0 100644
--- src/java/org/apache/nutch/util/URLUtil.java
+++ src/java/org/apache/nutch/util/URLUtil.java
@@ -383,9 +383,15 @@ public class URLUtil {
   public static String toASCII(String url) {
     try {
       URL u = new URL(url);
+      String host = u.getHost();
+      if (host == null || host.isEmpty()) {
+        // no host name => no punycoded domain name
+        // also do not add additional slashes for file: URLs (NUTCH-1880)
+        return url;
+      }
       URI p = new URI(u.getProtocol(),
         u.getUserInfo(),
-        IDN.toASCII(u.getHost()),
+        IDN.toASCII(host),
         u.getPort(),
         u.getPath(),
         u.getQuery(),
@@ -401,6 +407,12 @@ public class URLUtil {
   public static String toUNICODE(String url) {
     try {
       URL u = new URL(url);
+      String host = u.getHost();
+      if (host == null || host.isEmpty()) {
+        // no host name => no punycoded domain name
+        // also do not add additional slashes for file: URLs (NUTCH-1880)
+        return url;
+      }
       StringBuilder sb = new StringBuilder();
       sb.append(u.getProtocol());
       sb.append("://");
@@ -408,7 +420,7 @@ public class URLUtil {
         sb.append(u.getUserInfo());
         sb.append('@');
       }
-      sb.append(IDN.toUnicode(u.getHost()));
+      sb.append(IDN.toUnicode(host));
       if (u.getPort() != -1) {
         sb.append(':');
         sb.append(u.getPort());
diff --git src/test/org/apache/nutch/util/TestURLUtil.java src/test/org/apache/nutch/util/TestURLUtil.java
index 9550709..bca6ab2 100644
--- src/test/org/apache/nutch/util/TestURLUtil.java
+++ src/test/org/apache/nutch/util/TestURLUtil.java
@@ -277,4 +277,11 @@ public class TestURLUtil {
         URLUtil.toASCII("http://www.medizin.uni-tübingen.de:8080/search.php?q=abc#p1")); 
   }
 
+  @Test
+  public void testFileProtocol() throws Exception {
+    // keep one single slash NUTCH-XXX
+    assertEquals("file:/path/file.html", URLUtil.toASCII("file:/path/file.html"));
+    assertEquals("file:/path/file.html", URLUtil.toUNICODE("file:/path/file.html"));
+  }
+
 }