2 files changed, 10 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;
+    }
 }