3 files changed, 23 insertions, 0 deletions
diff --git a/chapter13/src/main/java/com/seaofnodes/simple/node/AddNode.java b/chapter13/src/main/java/com/seaofnodes/simple/node/AddNode.java
index 48350957..67eacd3a 100644
--- a/chapter13/src/main/java/com/seaofnodes/simple/node/AddNode.java
+++ b/chapter13/src/main/java/com/seaofnodes/simple/node/AddNode.java
@@ -173,4 +173,9 @@ public class AddNode extends Node {
 
     @Override Node copy(Node lhs, Node rhs) { return new AddNode(lhs,rhs); }
     @Override Node copyF() { return new AddFNode(null,null); }
+    @Override public String err() {
+        if( !(in(1)._type instanceof TypeInteger) ) return "Cannot '"+label()+"' " + in(1)._type;
+        if( !(in(2)._type instanceof TypeInteger) ) return "Cannot '"+label()+"' " + in(2)._type;
+        return null;
+    }
 }
diff --git a/chapter13/src/main/java/com/seaofnodes/simple/node/MulNode.java b/chapter13/src/main/java/com/seaofnodes/simple/node/MulNode.java
index 8c8a6e0a..16915a1c 100644
--- a/chapter13/src/main/java/com/seaofnodes/simple/node/MulNode.java
+++ b/chapter13/src/main/java/com/seaofnodes/simple/node/MulNode.java
@@ -55,4 +55,9 @@ public class MulNode extends Node {
     }
     @Override Node copy(Node lhs, Node rhs) { return new MulNode(lhs,rhs); }
     @Override Node copyF() { return new MulFNode(null,null); }
+    @Override public String err() {
+        if( !(in(1)._type instanceof TypeInteger) ) return "Cannot '"+label()+"' " + in(1)._type;
+        if( !(in(2)._type instanceof TypeInteger) ) return "Cannot '"+label()+"' " + in(2)._type;
+        return null;
+    }
 }
diff --git a/chapter13/src/test/java/com/seaofnodes/simple/Chapter13Test.java b/chapter13/src/test/java/com/seaofnodes/simple/Chapter13Test.java
index b425fd85..59f08963 100644
--- a/chapter13/src/test/java/com/seaofnodes/simple/Chapter13Test.java
+++ b/chapter13/src/test/java/com/seaofnodes/simple/Chapter13Test.java
@@ -131,6 +131,19 @@ return n.i;
     }
 
     @Test
+    public void testNullRef4() {
+        Parser parser = new Parser("-null-5/null-5");
+        try { parser.parse().iterate(); fail(); }
+        catch( Exception e ) { assertEquals("Expected an identifier, found 'null'",e.getMessage()); }
+    }
+
+    @Test public void testNullRef5() {
+        Parser parser = new Parser("return null+42;");
+        try { parser.parse().iterate(); fail(); }
+        catch( Exception e ) { assertEquals("Cannot 'Add' null",e.getMessage()); }
+    }
+
+    @Test
     public void testEmpty() {
         Parser parser = new Parser(
 """