diff --git a/org.jacoco.core.test/src/org/jacoco/core/internal/instr/InstrSupportTest.java b/org.jacoco.core.test/src/org/jacoco/core/internal/instr/InstrSupportTest.java
index 95de8bd..b8a89cb 100644
--- a/org.jacoco.core.test/src/org/jacoco/core/internal/instr/InstrSupportTest.java
+++ b/org.jacoco.core.test/src/org/jacoco/core/internal/instr/InstrSupportTest.java
@@ -17,7 +17,9 @@
 
 import org.jacoco.core.internal.BytecodeVersion;
 import org.junit.Before;
+import org.junit.Rule;
 import org.junit.Test;
+import org.junit.rules.ExpectedException;
 import org.objectweb.asm.Opcodes;
 import org.objectweb.asm.util.Printer;
 import org.objectweb.asm.util.Textifier;
@@ -31,6 +33,9 @@
 	private Printer printer;
 	private TraceMethodVisitor trace;
 
+	@Rule
+	public ExpectedException exception = ExpectedException.none();
+
 	@Before
 	public void setup() {
 		printer = new Textifier();
@@ -56,17 +61,25 @@
 	}
 
 	@Test
-	public void testAssertNotIntrumentedPositive() {
+	public void assertNotIntrumented_should_accept_non_jacoco_memebers() {
 		InstrSupport.assertNotInstrumented("run", "Foo");
 	}
 
-	@Test(expected = IllegalStateException.class)
-	public void testAssertNotIntrumentedField() {
+	@Test
+	public void assertNotIntrumented_should_throw_exception_when_jacoco_data_field_is_present() {
+		exception.expect(IllegalStateException.class);
+		exception.expectMessage(
+				"Cannot process instrumented class Foo. Please supply original non-instrumented classes.");
+
 		InstrSupport.assertNotInstrumented("$jacocoData", "Foo");
 	}
 
-	@Test(expected = IllegalStateException.class)
-	public void testAssertNotIntrumentedMethod() {
+	@Test
+	public void assertNotIntrumented_should_throw_exception_when_jacoco_init_method_is_present() {
+		exception.expect(IllegalStateException.class);
+		exception.expectMessage(
+				"Cannot process instrumented class Foo. Please supply original non-instrumented classes.");
+
 		InstrSupport.assertNotInstrumented("$jacocoInit", "Foo");
 	}
 
diff --git a/org.jacoco.core/src/org/jacoco/core/internal/instr/InstrSupport.java b/org.jacoco.core/src/org/jacoco/core/internal/instr/InstrSupport.java
index c7b7cf8..ed52bd3 100644
--- a/org.jacoco.core/src/org/jacoco/core/internal/instr/InstrSupport.java
+++ b/org.jacoco.core/src/org/jacoco/core/internal/instr/InstrSupport.java
@@ -185,8 +185,9 @@
 	public static void assertNotInstrumented(final String member,
 			final String owner) throws IllegalStateException {
 		if (member.equals(DATAFIELD_NAME) || member.equals(INITMETHOD_NAME)) {
-			throw new IllegalStateException(
-					format("Class %s is already instrumented.", owner));
+			throw new IllegalStateException(format(
+					"Cannot process instrumented class %s. Please supply original non-instrumented classes.",
+					owner));
 		}
 	}
 
diff --git a/org.jacoco.doc/docroot/doc/changes.html b/org.jacoco.doc/docroot/doc/changes.html
index ce2475c..5299b4e 100644
--- a/org.jacoco.doc/docroot/doc/changes.html
+++ b/org.jacoco.doc/docroot/doc/changes.html
@@ -38,6 +38,13 @@
       (GitHub <a href="https://github.com/jacoco/jacoco/issues/672">#672</a>).</li>
 </ul>
 
+<h3>Non-functional Changes</h3>
+<ul>
+  <li>Improved error message when already instrumented classes are used for
+      instrumentation or analysis
+      (GitHub <a href="https://github.com/jacoco/jacoco/issues/703">#703</a>).</li>
+</ul>
+
 <h2>Release 0.8.1 (2018/03/21)</h2>
 
 <h3>New Features</h3>