diff --git a/docs/syntax.md b/docs/syntax.md
index 7dbc6f8..005883a 100644
--- a/docs/syntax.md
+++ b/docs/syntax.md
@@ -329,7 +329,7 @@ Alternatively items can be added to just the BOM by putting them in the section
   manufacturer: <str>  # manufacturer name  
 ```
 
-## Tweak entries
+## GraphViz tweaking (experimental)
 
 ```yaml
   # Optional tweaking of the .gv output.
@@ -342,15 +342,14 @@ Alternatively items can be added to just the BOM by putting them in the section
     # The leading string might be in "quotes" in 
     # the .gv output. This leading string must be
     # followed by attributes in [square brackets].
-    # Entries containing HTML in an attribute are
+    # Entries with an attribute containing HTML are
     # not supported.
     <str>:  # leading string of .gv entry
       <str> : <str>  # attribute and its new value
       # Any number of attributes can be overridden
       # for each entry
 
-  append:  # single or list of .gv entries to append
-    <str>  # strings to append might have multiple lines
+  append: <str/list> # string or list of strings to append to the .gv output
 ```
 
 ## Colors
diff --git a/src/wireviz/Harness.py b/src/wireviz/Harness.py
index 1d6c586..548a687 100644
--- a/src/wireviz/Harness.py
+++ b/src/wireviz/Harness.py
@@ -3,7 +3,7 @@
 
 from graphviz import Graph
 from collections import Counter
-from typing import List, Union
+from typing import Any, List, Union
 from dataclasses import dataclass
 from pathlib import Path
 from itertools import zip_longest
@@ -345,9 +345,9 @@ class Harness:
             dot.node(cable.name, label=f'<\n{html}\n>', shape='box',
                      style=style, fillcolor=translate_color(bgcolor, "HEX"))
 
-        def typecheck(name: str, var, type) -> None:
-            if not isinstance(var, type):
-                raise Exception(f'Unexpected value type of {name}: {var}')
+        def typecheck(name: str, value: Any, expect: type) -> None:
+            if not isinstance(value, expect):
+                raise Exception(f'Unexpected value type of {name}: Expected {expect}, got {type(value)}\n{value}')
 
         # TODO?: Differ between override attributes and HTML?
         if self.tweak.override is not None:
@@ -380,7 +380,7 @@ class Harness:
                     typecheck(f'tweak.append[{i}]', element, str)
                 dot.body.extend(self.tweak.append)
             else:
-                typecheck(f'tweak.append', self.tweak.append, str)
+                typecheck('tweak.append', self.tweak.append, str)
                 dot.body.append(self.tweak.append)
 
         return dot