Index: src/test/java/org/quartz/CronExpressionTest.java
===================================================================
--- src/test/java/org/quartz/CronExpressionTest.java	(revision 691)
+++ src/test/java/org/quartz/CronExpressionTest.java	(working copy)
@@ -100,4 +100,25 @@
         newExpression.getNextValidTimeAfter(new Date());
     }
 
+    /*
+     * QUARTZ-574: Showing that storeExpressionVals correctly calculates the month number
+     */
+    public void testQuartz574() {
+        try {
+            CronExpression cronExpression = new CronExpression("* * * * Foo ? ");
+            fail("Expected ParseException did not fire for non-existent month");
+        } catch(ParseException pe) {
+            assertTrue("Incorrect ParseException thrown", 
+                pe.getMessage().startsWith("Invalid Month value:"));
+        }
+
+        try {
+            CronExpression cronExpression = new CronExpression("* * * * Jan-Foo ? ");
+            fail("Expected ParseException did not fire for non-existent month");
+        } catch(ParseException pe) {
+            assertTrue("Incorrect ParseException thrown", 
+                pe.getMessage().startsWith("Invalid Month value:"));
+        }
+    }
+
 }
Index: src/java/org/quartz/CronExpression.java
===================================================================
--- src/java/org/quartz/CronExpression.java	(revision 691)
+++ src/java/org/quartz/CronExpression.java	(working copy)
@@ -446,7 +446,7 @@
             int eval = -1;
             if (type == MONTH) {
                 sval = getMonthNumber(sub) + 1;
-                if (sval < 0) {
+                if (sval <= 0) {
                     throw new ParseException("Invalid Month value: '" + sub + "'", i);
                 }
                 if (s.length() > i + 3) {
@@ -455,7 +455,7 @@
                         i += 4;
                         sub = s.substring(i, i + 3);
                         eval = getMonthNumber(sub) + 1;
-                        if (eval < 0) {
+                        if (eval <= 0) {
                             throw new ParseException("Invalid Month value: '" + sub + "'", i);
                         }
                     }