diff --git a/webmagic-core/src/main/java/us/codecraft/webmagic/annotation/ExtractBy.java b/webmagic-core/src/main/java/us/codecraft/webmagic/annotation/ExtractBy.java
index 4c791fd..2d08417 100644
--- a/webmagic-core/src/main/java/us/codecraft/webmagic/annotation/ExtractBy.java
+++ b/webmagic-core/src/main/java/us/codecraft/webmagic/annotation/ExtractBy.java
@@ -13,6 +13,8 @@ import java.lang.annotation.Target;
 @Target({ElementType.FIELD})
 public @interface ExtractBy {
 
+
+    //TODO: add list support
     String value();
 
     public enum Type {XPath, Regex, Css};
diff --git a/webmagic-core/src/main/java/us/codecraft/webmagic/annotation/PageModelExtractor.java b/webmagic-core/src/main/java/us/codecraft/webmagic/annotation/PageModelExtractor.java
index 41f635c..e610e10 100644
--- a/webmagic-core/src/main/java/us/codecraft/webmagic/annotation/PageModelExtractor.java
+++ b/webmagic-core/src/main/java/us/codecraft/webmagic/annotation/PageModelExtractor.java
@@ -42,6 +42,9 @@ class PageModelExtractor {
         fieldExtractors = new ArrayList<FieldExtractor>();
         for (Field field : clazz.getDeclaredFields()) {
             field.setAccessible(true);
+            if (!field.getType().isAssignableFrom(String.class)){
+                 throw new IllegalStateException("Field "+field.getName()+" must be string");
+            }
             ExtractBy extractBy = field.getAnnotation(ExtractBy.class);
             if (extractBy != null) {
                 String value = extractBy.value();
diff --git a/webmagic-samples/src/main/java/us/codecraft/webmagic/annotation/samples/IteyeBlog.java b/webmagic-samples/src/main/java/us/codecraft/webmagic/annotation/samples/IteyeBlog.java
index 09a1d5b..48e4129 100644
--- a/webmagic-samples/src/main/java/us/codecraft/webmagic/annotation/samples/IteyeBlog.java
+++ b/webmagic-samples/src/main/java/us/codecraft/webmagic/annotation/samples/IteyeBlog.java
@@ -14,7 +14,7 @@ import us.codecraft.webmagic.annotation.TargetUrl;
 public class IteyeBlog implements Blog{
 
     @ExtractBy("//title")
-    private String title;
+    private int title;
 
     @ExtractBy(value = "div#blog_content",type = ExtractBy.Type.Css)
     private String content;
@@ -32,7 +32,7 @@ public class IteyeBlog implements Blog{
     }
 
     public String getTitle() {
-        return title;
+        return null;
     }
 
     public String getContent() {