4 files changed, 25 insertions, 6 deletions
diff --git a/chapter17/src/main/java/com/seaofnodes/simple/node/AddNode.java b/chapter17/src/main/java/com/seaofnodes/simple/node/AddNode.java
index 0834a381..4007df91 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/node/AddNode.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/node/AddNode.java
@@ -44,6 +44,7 @@ public class AddNode extends Node {
     public Node idealize () {
         Node lhs = in(1);
         Node rhs = in(2);
+        if( rhs.err() != null ) return null;
         Type t2 = rhs._type;
 
         // Add of 0.  We do not check for (0+x) because this will already
@@ -184,4 +185,9 @@ 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/chapter17/src/main/java/com/seaofnodes/simple/node/MulNode.java b/chapter17/src/main/java/com/seaofnodes/simple/node/MulNode.java
index b5ef91e9..949af27a 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/node/MulNode.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/node/MulNode.java
@@ -60,4 +60,9 @@ 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/chapter17/src/test/java/com/seaofnodes/simple/Chapter13Test.java b/chapter17/src/test/java/com/seaofnodes/simple/Chapter13Test.java
index 30d9b147..eeb28f1d 100644
--- a/chapter17/src/test/java/com/seaofnodes/simple/Chapter13Test.java
+++ b/chapter17/src/test/java/com/seaofnodes/simple/Chapter13Test.java
@@ -130,6 +130,19 @@ return n.i;
     }
 
     @Test
+    public void testNullRef4() {
+        Parser parser = new Parser("-null-5/null-5;");
+        try { parser.parse().iterate(); fail(); }
+        catch( Exception e ) { assertEquals("Cannot 'Add' 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(
 """
diff --git a/chapter17/src/test/java/com/seaofnodes/simple/Chapter17Test.java b/chapter17/src/test/java/com/seaofnodes/simple/Chapter17Test.java
index 4a7e959a..1ca9a515 100644
--- a/chapter17/src/test/java/com/seaofnodes/simple/Chapter17Test.java
+++ b/chapter17/src/test/java/com/seaofnodes/simple/Chapter17Test.java
@@ -11,12 +11,7 @@ public class Chapter17Test {
 
     @Test
     public void testJig() {
-        Parser parser = new Parser("""
-int i = 0;
-i=i=1;
-return i;
-                                   //return 3.14;
-""");
+        Parser parser = new Parser("return 1;");
         StopNode stop = parser.parse().iterate();
         assertEquals("return 1;", stop.toString());
         assertEquals(1L, Evaluator.evaluate(stop,  0));