--- src/java/org/apache/poi/ss/formula/functions/WeekNum.java	(revision 1784169)
+++ src/java/org/apache/poi/ss/formula/functions/WeekNum.java	(working copy)
@@ -27,6 +27,8 @@ 
 import org.apache.poi.ss.formula.eval.ValueEval;
 import org.apache.poi.ss.usermodel.DateUtil;
 import org.apache.poi.util.LocaleUtil;
+import org.apache.poi.util.POILogger;
+import org.apache.poi.util.POILogFactory;
 
 /**
  * Implementation for Excel WeekNum() function.<p/>
@@ -43,9 +45,17 @@ 
  * 1	Week begins on Sunday. Weekdays are numbered 1 through 7.
  * 2	Week begins on Monday. Weekdays are numbered 1 through 7.
  */
-public class WeekNum extends Fixed2ArgFunction implements FreeRefFunction {
+public class WeekNum extends Var1or2ArgFunction implements FreeRefFunction {
+    private static final POILogger LOGGER = POILogFactory.getLogger(WeekNum.class);
     public static final FreeRefFunction instance = new WeekNum();
+    private static final ValueEval DEFAULT_RETURN_TYPE = new NumberEval(1);
 
+    @Override
+    public ValueEval evaluate(int srcRowIndex, int srcColumnIndex, ValueEval serialNumVE) {
+        return evaluate(srcRowIndex, srcColumnIndex, serialNumVE, DEFAULT_RETURN_TYPE);
+    }
+
+    @Override
     public ValueEval evaluate(int srcRowIndex, int srcColumnIndex, ValueEval serialNumVE, ValueEval returnTypeVE) {
         double serialNum = 0.0;
         try {
@@ -80,10 +90,17 @@ 
         return cal.get(Calendar.WEEK_OF_YEAR);
     }
 
+    @Override
     public ValueEval evaluate(ValueEval[] args, OperationEvaluationContext ec) {
+        if (args.length == 1) {
+            return evaluate(ec.getRowIndex(), ec.getColumnIndex(), args[0], DEFAULT_RETURN_TYPE);
+        }
         if (args.length == 2) {
             return evaluate(ec.getRowIndex(), ec.getColumnIndex(), args[0], args[1]);
         }
+        if (LOGGER.check(POILogger.WARN)) {
+            LOGGER.log(POILogger.WARN, "WeekNum() expected 1-2 args, got " + args.length);
+        }
         return ErrorEval.VALUE_INVALID;
     }
 }