diff --git a/xmlschema-core/src/main/java/org/apache/ws/commons/schema/EnumUtil.java b/xmlschema-core/src/main/java/org/apache/ws/commons/schema/EnumUtil.java
index aa79127a..e5a3a3ff 100644
--- a/xmlschema-core/src/main/java/org/apache/ws/commons/schema/EnumUtil.java
+++ b/xmlschema-core/src/main/java/org/apache/ws/commons/schema/EnumUtil.java
@@ -30,7 +30,12 @@ private EnumUtil() {
     }
     
     static <T extends Enum<T>> T valueOf(Class<T> enumClass, String name) {
-        return Enum.valueOf(enumClass, name.toUpperCase(Locale.ENGLISH));
+        try {
+            return Enum.valueOf(enumClass, name.toUpperCase(Locale.ENGLISH));
+        } catch (IllegalArgumentException e) {
+            throw new XmlSchemaException("Invalid value \"" + name + "\" for "
+                                         + enumClass.getSimpleName() + ".", e);
+        }
     }
 
 }
diff --git a/xmlschema-core/src/main/java/org/apache/ws/commons/schema/SchemaBuilder.java b/xmlschema-core/src/main/java/org/apache/ws/commons/schema/SchemaBuilder.java
index 66418f23..bbd215fa 100644
--- a/xmlschema-core/src/main/java/org/apache/ws/commons/schema/SchemaBuilder.java
+++ b/xmlschema-core/src/main/java/org/apache/ws/commons/schema/SchemaBuilder.java
@@ -584,7 +584,7 @@ public void validate(XmlSchema pSchema) {
                 if (isEmpty(uri)) {
                     valid = isEmpty(pSchema.getSyntacticalTargetNamespace());
                 } else {
-                    valid = pSchema.getSyntacticalTargetNamespace().equals(uri);
+                    valid = uri.equals(pSchema.getSyntacticalTargetNamespace());
                 }
                 if (!valid) {
                     throw new XmlSchemaException("An imported schema was announced to have the namespace "
@@ -931,7 +931,7 @@ private QName getRefQName(String pName, NamespaceContext pContext) {
             }
 
             if (uri == null || Constants.NULL_NS_URI.equals(uri)) {
-                throw new IllegalStateException("The prefix " + prefix + " is not bound.");
+                throw new XmlSchemaException("The prefix " + prefix + " is not bound.");
             }
             localName = pName.substring(offset + 1);
         }
@@ -1355,6 +1355,10 @@ private XmlSchemaComplexContentRestriction handleComplexContentRestriction(XmlSc
             }
 
             if (constraintEl.hasAttribute("refer")) {
+                if (!(constraint instanceof XmlSchemaKeyref)) {
+                    throw new XmlSchemaException("A \"refer\" attribute is only permitted on xs:keyref,"
+                                                 + " not on xs:" + constraintEl.getLocalName() + ".");
+                }
                 String name = constraintEl.getAttribute("refer");
                 ((XmlSchemaKeyref)constraint).refer = getRefQName(name, constraintEl);
             }
diff --git a/xmlschema-core/src/main/java/org/apache/ws/commons/schema/XmlSchemaCollection.java b/xmlschema-core/src/main/java/org/apache/ws/commons/schema/XmlSchemaCollection.java
index dfe59fb3..1383ee30 100644
--- a/xmlschema-core/src/main/java/org/apache/ws/commons/schema/XmlSchemaCollection.java
+++ b/xmlschema-core/src/main/java/org/apache/ws/commons/schema/XmlSchemaCollection.java
@@ -714,10 +714,10 @@ public String toString() {
     void addSchema(SchemaKey pKey, XmlSchema pSchema) {
         if (schemas.containsKey(pKey)) {
             throw 
-                new IllegalStateException("A schema with target namespace " 
-                                          + pKey.getNamespace()
-                                          + " and system ID " 
-                                          + pKey.getSystemId() + " is already present.");
+                new XmlSchemaException("A schema with target namespace "
+                                       + pKey.getNamespace()
+                                       + " and system ID "
+                                       + pKey.getSystemId() + " is already present.");
         }
         schemas.put(pKey, pSchema);
     }
diff --git a/xmlschema-core/src/test/java/tests/ExceptionContractTest.java b/xmlschema-core/src/test/java/tests/ExceptionContractTest.java
new file mode 100644
index 00000000..5e029f05
--- /dev/null
+++ b/xmlschema-core/src/test/java/tests/ExceptionContractTest.java
@@ -0,0 +1,82 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package tests;
+
+import java.io.StringReader;
+
+import org.apache.ws.commons.schema.XmlSchemaCollection;
+import org.apache.ws.commons.schema.XmlSchemaException;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class ExceptionContractTest extends Assert {
+
+    private void assertRejectedWithXmlSchemaException(String schemaBody) {
+        String schema =
+            "<xs:schema xmlns:xs=\"http://www.w3.org/2001/XMLSchema\""
+            + " targetNamespace=\"urn:contract\">" + schemaBody + "</xs:schema>";
+        XmlSchemaCollection collection = new XmlSchemaCollection();
+        try {
+            collection.read(new StringReader(schema));
+            fail("The crafted schema should have been rejected: " + schemaBody);
+        } catch (XmlSchemaException expected) {
+            // Expected documented failure surface.
+        }
+    }
+
+    @Test
+    public void testUndeclaredPrefixInTypeReference() {
+        assertRejectedWithXmlSchemaException("<xs:element name=\"a\" type=\"nope:T\"/>");
+    }
+
+    @Test
+    public void testReferOnKeyConstraint() {
+        assertRejectedWithXmlSchemaException(
+            "<xs:element name=\"a\" type=\"xs:string\">"
+            + "<xs:key name=\"k\" refer=\"xs:b\">"
+            + "<xs:selector xpath=\".\"/><xs:field xpath=\"@id\"/>"
+            + "</xs:key></xs:element>");
+    }
+
+    @Test
+    public void testInvalidFormAttributeValue() {
+        assertRejectedWithXmlSchemaException(
+            "<xs:element name=\"a\"><xs:complexType>"
+            + "<xs:attribute name=\"x\" form=\"bogus\"/>"
+            + "</xs:complexType></xs:element>");
+    }
+
+    @Test
+    public void testInvalidUseAttributeValue() {
+        assertRejectedWithXmlSchemaException(
+            "<xs:element name=\"a\"><xs:complexType>"
+            + "<xs:attribute name=\"x\" use=\"bogus\"/>"
+            + "</xs:complexType></xs:element>");
+    }
+
+    @Test
+    public void testInvalidProcessContentsValue() {
+        assertRejectedWithXmlSchemaException(
+            "<xs:element name=\"a\"><xs:complexType><xs:sequence>"
+            + "<xs:any processContents=\"bogus\"/>"
+            + "</xs:sequence></xs:complexType></xs:element>");
+    }
+}
\ No newline at end of file