diff --git a/src/sixtypical/parser.py b/src/sixtypical/parser.py
index 7865482..063b227 100644
--- a/src/sixtypical/parser.py
+++ b/src/sixtypical/parser.py
@@ -82,8 +82,14 @@ class Parser(object):
     def __init__(self, text):
         self.scanner = Scanner(text)
         self.symbols = {}  # token -> SymEntry
+        for token in ('a', 'x', 'y'):
+            self.symbols[token] = SymEntry(None, LocationRef(TYPE_BYTE, token))
+        for token in ('c', 'z', 'n', 'v'):
+            self.symbols[token] = SymEntry(None, LocationRef(TYPE_BIT, token))
 
     def lookup(self, name):
+        if name not in self.symbols:
+            raise SyntaxError('Undefined symbol "%s"' % name)
         return self.symbols[name].model
 
     def program(self):
@@ -93,14 +99,14 @@ class Parser(object):
             defn = self.defn()
             name = defn.name
             if name in self.symbols:
-                raise KeyError(name)
+                raise SyntaxError(name)
             self.symbols[name] = SymEntry(defn, LocationRef(TYPE_BYTE, name))
             defns.append(defn)
         while self.scanner.on('routine'):
             routine = self.routine()
             name = routine.name
             if name in self.symbols:
-                raise KeyError(name)
+                raise SyntaxError(name)
             self.symbols[name] = SymEntry(routine, None)
             routines.append(routine)
         self.scanner.check_type('EOF')
@@ -151,15 +157,7 @@ class Parser(object):
         return accum
 
     def locexpr(self):
-        if self.scanner.token in ('a', 'x', 'y'):
-            loc = LocationRef(TYPE_BYTE, self.scanner.token)
-            self.scanner.scan()
-            return loc
-        elif self.scanner.token in ('c', 'z', 'n', 'v'):
-            loc = LocationRef(TYPE_BIT, self.scanner.token)
-            self.scanner.scan()
-            return loc
-        elif self.scanner.token in ('on', 'off'):
+        if self.scanner.token in ('on', 'off'):
             loc = ConstantRef(TYPE_BIT, 1 if self.scanner.token == 'on' else 0)
             self.scanner.scan()
             return loc
diff --git a/tests/SixtyPical Execution.md b/tests/SixtyPical Execution.md
index 6eab66f..a9d8bf8 100644
--- a/tests/SixtyPical Execution.md	
+++ b/tests/SixtyPical Execution.md	
@@ -45,25 +45,6 @@ Program accesses a memory location.
     = y: 0
     = z: 0
 
-Can't access an undeclared memory location.
-
-    | routine main {
-    |     ld a, 0
-    |     st a, lives
-    | }
-    ? KeyError
-
-Can't define two memory locations with the same name.
-
-    | byte lives
-    | byte lives
-    | 
-    | routine main {
-    |     ld a, 0
-    |     st a, lives
-    | }
-    ? KeyError
-
 Add honours carry.
 
     | routine main {
@@ -294,28 +275,6 @@ Call routine.
     = y: 3
     = z: 0
 
-Can't call routine that hasn;t been defined.
-
-    | routine main {
-    |     ld x, 0
-    |     ld y, 1
-    |     call up
-    |     call up
-    | }
-    ? KeyError
-
-Can't define two routines with the same name.
-
-    | routine main {
-    |     inc x
-    |     inc y
-    | }
-    | routine main {
-    |     ld x, 0
-    |     ld y, 1
-    | }
-    ? KeyError
-
 If.
 
     | routine main {
diff --git a/tests/SixtyPical Syntax.md b/tests/SixtyPical Syntax.md
index 00713f5..d1df10d 100644
--- a/tests/SixtyPical Syntax.md	
+++ b/tests/SixtyPical Syntax.md	
@@ -111,3 +111,58 @@ Extern memory locations
     |   st a, screen
     | }
     = ok
+
+Can't access an undeclared memory location.
+
+    | routine main {
+    |     ld a, 0
+    |     st a, lives
+    | }
+    ? SyntaxError
+
+Can't define two memory locations with the same name.
+
+    | byte lives
+    | byte lives
+    | 
+    | routine main {
+    |     ld a, 0
+    |     st a, lives
+    | }
+    ? SyntaxError
+
+Can't shadow the name of a register or a flag.
+
+    | byte a
+    | 
+    | routine main {
+    | }
+    ? SyntaxError
+
+    | byte z
+    | 
+    | routine main {
+    | }
+    ? SyntaxError
+
+> Can't call routine that hasn;t been defined.
+> 
+>     | routine main {
+>     |     ld x, 0
+>     |     ld y, 1
+>     |     call up
+>     |     call up
+>     | }
+>     ? SyntaxError
+
+Can't define two routines with the same name.
+
+    | routine main {
+    |     inc x
+    |     inc y
+    | }
+    | routine main {
+    |     ld x, 0
+    |     ld y, 1
+    | }
+    ? SyntaxError