diff --git a/src/main/java/pythagoras/f/NoninvertibleTransformException.java b/src/main/java/pythagoras/f/NoninvertibleTransformException.java
index 0286276..ffe37fe 100644
--- a/src/main/java/pythagoras/f/NoninvertibleTransformException.java
+++ b/src/main/java/pythagoras/f/NoninvertibleTransformException.java
@@ -4,6 +4,10 @@
 
 package pythagoras.f;
 
+/**
+ * An exception thrown by {@link AffineTransform} when a request for an inverse transform cannot be
+ * satisfied.
+ */
 public class NoninvertibleTransformException extends java.lang.Exception
 {
     public NoninvertibleTransformException (String s) {