diff --git a/spring-binding/src/main/java/org/springframework/binding/convert/ConversionException.java b/spring-binding/src/main/java/org/springframework/binding/convert/ConversionException.java
index b1a78505..054742f0 100644
--- a/spring-binding/src/main/java/org/springframework/binding/convert/ConversionException.java
+++ b/spring-binding/src/main/java/org/springframework/binding/convert/ConversionException.java
@@ -103,6 +103,20 @@ public class ConversionException extends NestedRuntimeException {
 		this.targetClass = null; // not available
 	}
 	
+	/**
+	 * Creates a new conversion exception.
+	 * @param sourceClass the source type
+	 * @param value the value we tried to convert
+	 * @param targetClass the target type
+	 * @param message a descriptive message
+	 */
+	public ConversionException(Class sourceClass, Object value, Class targetClass, String message) {
+		super(message);
+		this.sourceClass = sourceClass;
+		this.value = value;
+		this.targetClass = targetClass;
+	}
+	
 	/**
 	 * Returns the source type.
 	 */
diff --git a/spring-binding/src/main/java/org/springframework/binding/convert/ConversionExecutor.java b/spring-binding/src/main/java/org/springframework/binding/convert/ConversionExecutor.java
index 53efce5e..f8de988e 100644
--- a/spring-binding/src/main/java/org/springframework/binding/convert/ConversionExecutor.java
+++ b/spring-binding/src/main/java/org/springframework/binding/convert/ConversionExecutor.java
@@ -98,8 +98,13 @@ public class ConversionExecutor {
 	 * behavior of the converter
 	 */
 	public Object execute(Object source, ConversionContext context) throws ConversionException {
-		if (source != null) {
-			Assert.isInstanceOf(sourceClass, source, "Not of source type: ");
+		if (getTargetClass().isInstance(source)) {
+			// source is already assignment compatible with target class
+			return source;
+		}
+		if (source != null && !getSourceClass().isInstance(source)) {
+			throw new ConversionException(getSourceClass(), source, getTargetClass(),
+					"Source object '" + source +  "' is expected to be a " + getSourceClass());
 		}
 		return converter.convert(source, targetClass, context);
 	}
diff --git a/spring-binding/src/test/java/org/springframework/binding/convert/ConversionExecutorTests.java b/spring-binding/src/test/java/org/springframework/binding/convert/ConversionExecutorTests.java
new file mode 100644
index 00000000..f6bfc89f
--- /dev/null
+++ b/spring-binding/src/test/java/org/springframework/binding/convert/ConversionExecutorTests.java
@@ -0,0 +1,73 @@
+/*
+ * Copyright 2004-2007 the original author or authors.
+ *
+ * Licensed 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 org.springframework.binding.convert;
+
+import java.util.Date;
+
+import junit.framework.TestCase;
+
+import org.springframework.binding.convert.support.AbstractConverter;
+
+/**
+ * Test case for {@link ConversionExecutor}.
+ */
+public class ConversionExecutorTests extends TestCase {
+	
+	private ConversionExecutor conversionExecutor;
+	
+	protected void setUp() throws Exception {
+		conversionExecutor = new ConversionExecutor(String.class, Date.class, new TestTextToDate());
+	}
+	
+	public void testTypeConversion() {
+		assertTrue(conversionExecutor.execute("10-10-2008").getClass().equals(Date.class));
+	}
+	
+	public void testAssignmentCompatibleTypeConversion() {
+		java.sql.Date date = new java.sql.Date(123L);
+		assertSame(date, conversionExecutor.execute(date));
+	}
+	
+	public void testConvertNull() {
+		assertNull(conversionExecutor.execute(null));
+	}
+	
+	public void testIllegalType() {
+		try {
+			conversionExecutor.execute(new StringBuffer());
+			fail();
+		}
+		catch (ConversionException e) {
+			// expected
+		}
+	}
+
+	private class TestTextToDate extends AbstractConverter {
+		
+		public Class[] getSourceClasses() {
+			return new Class[] { String.class };
+		}
+		
+		public Class[] getTargetClasses() {
+			return new Class[] { Date.class };
+		}
+
+		protected Object doConvert(Object source, Class targetClass, ConversionContext context) throws Exception {
+			return source == null ? null : new Date();
+		}
+	}
+
+}
diff --git a/spring-webflow/changelog.txt b/spring-webflow/changelog.txt
index 572a6cde..89f406e1 100644
--- a/spring-webflow/changelog.txt
+++ b/spring-webflow/changelog.txt
@@ -8,6 +8,7 @@ Changes in version 1.0.4 (13.06.2007)
 Package org.springframework.binding
 * Fixed serializability problems in ConversionException, EvaluationException and MethodInvocationException.
 * Added targetCollection(String) method to MappingBuilder.
+* ConversionExecutor now checks for assignment compatible types (SWF-337).
 
 Package org.springframework.webflow.action
 * Improved FormAction.setupForm() JavaDoc (SWF-325).