diff --git a/HISTORY.md b/HISTORY.md
index d91ba53..6fb438d 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -22,6 +22,8 @@ Castile 0.5
 *   Requesting the AST be dumped, will also dump the AST with
     type assignments, if an error occurs during type checking.
 *   Established an abstract base class for compiler backends.
+*   Fixed a bug where tagged values were being tagged again
+    during a cast from a union type to another union type.
 
 Castile 0.4
 -----------
diff --git a/TODO.md b/TODO.md
index ad03f89..6878dea 100644
--- a/TODO.md
+++ b/TODO.md
@@ -6,8 +6,10 @@ TODO
 call equal_tagged_value() when you find a union type when
 comparing structs deeply.  (C backend, probably others)
 
-There appears to be a bug with casting a union type to
-itself?  The tag in the tagged value is the union?
+Actually the idea of deep equality of structs is suspect.
+Especially if the visibility of those structs is limited.
+Instead users should write their own `equal` functions
+(possibly abstract, possibly only testing equivalence.)
 
 Name mangling for compilers (prepend with `_` most likely.)
 
diff --git a/eg/assoc.castile b/eg/assoc.castile
index cc994b9..ceea2a6 100644
--- a/eg/assoc.castile
+++ b/eg/assoc.castile
@@ -22,7 +22,7 @@ fun lookup(a: assoc, k: string) {
     return null as string|void
   }
   typecase n is assoc {
-    return lookup(n, k)
+    return lookup(n, k) as string|void
   }
 }
 
diff --git a/src/castile/backends/c.py b/src/castile/backends/c.py
index 8856345..3c328ad 100644
--- a/src/castile/backends/c.py
+++ b/src/castile/backends/c.py
@@ -348,9 +348,13 @@ def find_field(name):
             self.compile(ast.children[0])
             self.write('->%s' % ast.value)
         elif ast.tag == 'TypeCast':
-            self.write('tag("%s",(void *)' % str(ast.children[0].type))
-            self.compile(ast.children[0])
-            self.write(')')
+            # If the LHS is not already a union type, promote it to a tagged value.
+            if isinstance(ast.children[0].type, Union):
+                self.compile(ast.children[0])
+            else:
+                self.write('tag("%s",(void *)' % str(ast.children[0].type))
+                self.compile(ast.children[0])
+                self.write(')')
         elif ast.tag == 'TypeCase':
             self.write_indent('if (is_tag("%s",' % str(ast.children[1].type))
             self.compile(ast.children[0])
diff --git a/src/castile/backends/javascript.py b/src/castile/backends/javascript.py
index 1f86f6f..20482db 100644
--- a/src/castile/backends/javascript.py
+++ b/src/castile/backends/javascript.py
@@ -191,9 +191,13 @@ def compile(self, ast):
             self.compile(ast.children[0])
             self.write('.%s' % ast.value)
         elif ast.tag == 'TypeCast':
-            self.write("['%s'," % str(ast.children[0].type))
-            self.compile(ast.children[0])
-            self.write(']')
+            # If the LHS is not already a union type, promote it to a tagged value.
+            if isinstance(ast.children[0].type, Union):
+                self.compile(ast.children[0])
+            else:
+                self.write("['%s'," % str(ast.children[0].type))
+                self.compile(ast.children[0])
+                self.write(']')
         elif ast.tag == 'TypeCase':
             self.write('if (')
             self.compile(ast.children[0])
diff --git a/src/castile/backends/ruby.py b/src/castile/backends/ruby.py
index 101a1d9..e0d8911 100644
--- a/src/castile/backends/ruby.py
+++ b/src/castile/backends/ruby.py
@@ -1,4 +1,5 @@
 from castile.backends.base import BaseCompiler
+from castile.types import Union
 
 
 OPS = {
@@ -186,9 +187,13 @@ def compile(self, ast):
             self.compile(ast.children[0])
             self.write('["%s"]' % ast.value)
         elif ast.tag == 'TypeCast':
-            self.write("['%s'," % str(ast.children[0].type))
-            self.compile(ast.children[0])
-            self.write(']')
+            # If the LHS is not already a union type, promote it to a tagged value.
+            if isinstance(ast.children[0].type, Union):
+                self.compile(ast.children[0])
+            else:
+                self.write("['%s'," % str(ast.children[0].type))
+                self.compile(ast.children[0])
+                self.write(']')
         elif ast.tag == 'TypeCase':
             self.write_indent('if (')
             self.compile(ast.children[0])
diff --git a/src/castile/backends/stackmac.py b/src/castile/backends/stackmac.py
index 0e1315a..07ffb25 100644
--- a/src/castile/backends/stackmac.py
+++ b/src/castile/backends/stackmac.py
@@ -238,12 +238,14 @@ def compile(self, ast):
         elif ast.tag == 'TypeCast':
             self.compile(ast.children[0])
             t = str(ast.children[0].type)
-            self.out.write('; tag with "%s"\n' % t)
-            if self.size_of(ast.children[0].type) == 0:
-                # special case.  there is nothing on the stack
-                self.out.write('push 0\n')
-            tag = self.get_tag(t)
-            self.out.write('tag %d\n' % tag)
+            # If the LHS is not already a union type, promote it to a tagged value.
+            if not isinstance(ast.children[0].type, Union):
+                self.out.write('; tag with "%s"\n' % t)
+                if self.size_of(ast.children[0].type) == 0:
+                    # special case.  there is nothing on the stack
+                    self.out.write('push 0\n')
+                tag = self.get_tag(t)
+                self.out.write('tag %d\n' % tag)
         elif ast.tag == 'TypeCase':
             end_typecase = self.get_label('end_typecase')
             self.compile(ast.children[0])
diff --git a/src/castile/eval.py b/src/castile/eval.py
index c7b9d02..c92d7bb 100644
--- a/src/castile/eval.py
+++ b/src/castile/eval.py
@@ -142,7 +142,9 @@ def eval(self, ast=None):
             return v
         elif ast.tag == 'TypeCast':
             v = self.eval(ast.children[0])
-            return TaggedValue(typeof(v), v)
+            if not isinstance(v, TaggedValue):
+                v = TaggedValue(typeof(v), v)
+            return v
         elif ast.tag == 'TypeCase':
             r = self.eval(ast.children[0])
             assert isinstance(r, TaggedValue)