diff --git a/src/ml/topic/common/preprocess/DataSetGen.java b/src/ml/topic/common/preprocess/DataSetGen.java
index e31e607..c5d1534 100644
--- a/src/ml/topic/common/preprocess/DataSetGen.java
+++ b/src/ml/topic/common/preprocess/DataSetGen.java
@@ -54,7 +54,7 @@ public static DataSet createFromNIPSCorpus(String filePath) throws IOException{
                             if (processedToken.equals(""))
                                 continue;
                             // 3. check if any invalid character is contained in the token
-                            if (TextUtil.isValid(processedToken)){
+                            if (!TextUtil.isValid(processedToken)){
                                 continue;
                             }
                             // 4. finally, remove the stop words. 
@@ -124,7 +124,7 @@ public static DataSet createFromNIPSCorpus(String filePath) throws IOException{
                             if (processedToken.equals(""))
                                 continue;
                             // 3. check if any invalid character is contained in the token
-                            if (TextUtil.isValid(processedToken)){
+                            if (!TextUtil.isValid(processedToken)){
                                 continue;
                             }
                             // 4. finally, remove the stop words. 
diff --git a/src/ml/topic/utils/TextUtil.java b/src/ml/topic/utils/TextUtil.java
index 569838d..529c05e 100644
--- a/src/ml/topic/utils/TextUtil.java
+++ b/src/ml/topic/utils/TextUtil.java
@@ -65,6 +65,6 @@ public static boolean isValid(String token){
             return false;
         }  
         
-        return false;
+        return true;
     }
 }