diff --git a/cmd/cue/cmd/help.go b/cmd/cue/cmd/help.go
index 399d545..67ec139 100644
--- a/cmd/cue/cmd/help.go
+++ b/cmd/cue/cmd/help.go
@@ -290,7 +290,9 @@
 Injecting values
 
 The injection mechanism allows values to be injected into fields
-that are marked with a "tag" attribute. For any field of the form
+that are not defined within the scope of a comprehension, list, or
+optional field and that are marked with a "tag" attribute. For any
+field of the form
 
    field: x @tag(key)
 
diff --git a/cmd/cue/cmd/testdata/script/inject.txt b/cmd/cue/cmd/testdata/script/inject.txt
new file mode 100644
index 0000000..b2f4403
--- /dev/null
+++ b/cmd/cue/cmd/testdata/script/inject.txt
@@ -0,0 +1,13 @@
+cue eval test.cue -t env=prod
+
+cmp stdout expect-stdout
+
+# TODO: report errors for invalid tags?
+
+-- test.cue --
+{
+    environment: "prod" | "staging" @tag(env,short=prod|staging)
+}
+
+-- expect-stdout --
+environment: "prod"
diff --git a/cmd/cue/cmd/testdata/script/injecterr.txt b/cmd/cue/cmd/testdata/script/injecterr.txt
new file mode 100644
index 0000000..f9bd857
--- /dev/null
+++ b/cmd/cue/cmd/testdata/script/injecterr.txt
@@ -0,0 +1,36 @@
+! cue eval test.cue -t env=prod
+
+cmp stderr expect-stderr
+
+# TODO: report errors for invalid tags?
+
+-- test.cue --
+{
+    environment: "prod" | "staging" @tag(env,short=prod|staging)
+
+    // Don't replace in optional
+    opt?: string @tag(env)
+    bulk: [string]: foo: string @tag(env)
+    bulk: x: {}
+
+    // Don't replace in lists.
+    a: [
+        { no_replace: string @tag(env) }
+    ]
+
+    // Don't allow in comprehensions
+    src: [1, 2]
+    for _ in src {
+        b: string @tag(prod)
+    }
+}
+
+-- expect-stderr --
+@tag not allowed within optional fields:
+    ./test.cue:5:18
+@tag not allowed within optional fields:
+    ./test.cue:6:33
+@tag not allowed within lists:
+    ./test.cue:11:30
+@tag not allowed within comprehension:
+    ./test.cue:17:19
diff --git a/cue/load/tags.go b/cue/load/tags.go
index d30b502..1417c0b 100644
--- a/cue/load/tags.go
+++ b/cue/load/tags.go
@@ -101,20 +101,34 @@
 //
 // TODO: should we limit the depth at which tags may occur?
 func findTags(b *build.Instance) (tags []tag, errs errors.Error) {
+	findInvalidTags := func(x ast.Node, msg string) {
+		ast.Walk(x, nil, func(n ast.Node) {
+			if f, ok := n.(*ast.Field); ok {
+				for _, a := range f.Attrs {
+					if key, _ := a.Split(); key == "tag" {
+						errs = errors.Append(errs, errors.Newf(a.Pos(), msg))
+						// TODO: add position of x.
+					}
+				}
+			}
+		})
+	}
 	for _, f := range b.Files {
 		ast.Walk(f, func(n ast.Node) bool {
-			if b.Err != nil {
-				return false
-			}
-
 			switch x := n.(type) {
-			case *ast.StructLit, *ast.File:
-				return true
+			case *ast.ListLit:
+				findInvalidTags(n, "@tag not allowed within lists")
+				return false
+
+			case *ast.Comprehension:
+				findInvalidTags(n, "@tag not allowed within comprehension")
+				return false
 
 			case *ast.Field:
 				// TODO: allow optional fields?
 				_, _, err := ast.LabelName(x.Label)
 				if err != nil || x.Optional != token.NoPos {
+					findInvalidTags(n, "@tag not allowed within optional fields")
 					return false
 				}
 
@@ -131,9 +145,8 @@
 					t.field = x
 					tags = append(tags, t)
 				}
-				return true
 			}
-			return false
+			return true
 		}, nil)
 	}
 	return tags, errs