diff --git a/cue/build.go b/cue/build.go
index f0fb2a5..4285d63 100644
--- a/cue/build.go
+++ b/cue/build.go
@@ -20,6 +20,7 @@
 	"encoding/gob"
 	"path"
 	"strconv"
+	"sync"
 
 	"cuelang.org/go/cue/ast"
 	"cuelang.org/go/cue/build"
@@ -33,8 +34,8 @@
 // Any operation that involves two Values or Instances should originate from
 // the same Runtime.
 type Runtime struct {
-	Context *build.Context // TODO: remove
-	idx     *index
+	ctx *build.Context // TODO: remove
+	idx *index
 }
 
 func dummyLoad(token.Pos, string) *build.Instance { return nil }
@@ -131,7 +132,7 @@
 // name in position information. The source may import builtin packages. Use
 // Build to allow importing non-builtin packages.
 func (r *Runtime) Compile(filename string, source interface{}) (*Instance, error) {
-	ctx := r.Context
+	ctx := r.ctx
 	if ctx == nil {
 		ctx = build.NewContext()
 	}
@@ -145,7 +146,7 @@
 // CompileFile compiles the given source file into an Instance. The source may
 // import builtin packages. Use Build to allow importing non-builtin packages.
 func (r *Runtime) CompileFile(file *ast.File) (*Instance, error) {
-	ctx := r.Context
+	ctx := r.ctx
 	if ctx == nil {
 		ctx = build.NewContext()
 	}
@@ -164,7 +165,7 @@
 // may import builtin packages. Use Build to allow importing non-builtin
 // packages.
 func (r *Runtime) CompileExpr(expr ast.Expr) (*Instance, error) {
-	ctx := r.Context
+	ctx := r.ctx
 	if ctx == nil {
 		ctx = build.NewContext()
 	}
@@ -250,6 +251,9 @@
 	offset label
 	parent *index
 	freeze bool
+
+	mutex     sync.Mutex
+	typeCache sync.Map // map[reflect.Type]evaluated
 }
 
 const sharedOffset = 0x40000000
diff --git a/cue/go.go b/cue/go.go
index e9435c3..5235f16 100644
--- a/cue/go.go
+++ b/cue/go.go
@@ -22,7 +22,6 @@
 	"reflect"
 	"sort"
 	"strings"
-	"sync"
 
 	"cuelang.org/go/cue/ast"
 	"cuelang.org/go/cue/parser"
@@ -381,16 +380,11 @@
 	return n
 }
 
-var (
-	typeCache sync.Map // map[reflect.Type]evaluated
-	mutex     sync.Mutex
-)
-
 func convertGoType(r *Runtime, t reflect.Type) value {
 	ctx := r.index().newContext()
 	// TODO: this can be much more efficient.
-	mutex.Lock()
-	defer mutex.Unlock()
+	ctx.mutex.Lock()
+	defer ctx.mutex.Unlock()
 	return goTypeToValue(ctx, true, t)
 }
 
@@ -405,7 +399,7 @@
 // TODO: if this value will always be unified with a concrete type in Go, then
 // many of the fields may be omitted.
 func goTypeToValue(ctx *context, allowNullDefault bool, t reflect.Type) (e value) {
-	if e, ok := typeCache.Load(t); ok {
+	if e, ok := ctx.typeCache.Load(t); ok {
 		return e.(value)
 	}
 
@@ -477,7 +471,7 @@
 		// resolve field tags to allow field tags to refer to the struct fields.
 		tags := map[label]string{}
 		obj := newStruct(baseValue{})
-		typeCache.Store(t, obj)
+		ctx.typeCache.Store(t, obj)
 
 		for i := 0; i < t.NumField(); i++ {
 			f := t.Field(i)
@@ -559,7 +553,7 @@
 store:
 	// TODO: store error if not nil?
 	if e != nil {
-		typeCache.Store(t, e)
+		ctx.typeCache.Store(t, e)
 	}
 	return e
 }