diff --git a/engine/references.go b/engine/references.go
index 3bb5c80..0f8594a 100644
--- a/engine/references.go
+++ b/engine/references.go
@@ -1,12 +1,14 @@
 package engine
 
-import "unsafe"
+import (
+	"math/rand"
+	"unsafe"
+)
 
 // ExternalReferences manages external references to be used by the Interpreter
 // to store references that can be passed to guest modules.
 type ExternalReferences struct {
-	nextid int32
-	refs   map[int32]uintptr
+	refs map[int32]uintptr
 }
 
 // NewReferences creates a new ExternalReferences store.
@@ -18,7 +20,7 @@ func NewReferences() *ExternalReferences {
 
 // Add adds a reference to the ExternalReferences store and returns the new reference id.
 func (r *ExternalReferences) Add(thing unsafe.Pointer) int32 {
-	id := r.referenceid()
+	id := r.newReferenceId()
 	r.refs[id] = uintptr(thing)
 	return id
 }
@@ -33,8 +35,17 @@ func (r *ExternalReferences) Remove(id int32) {
 	delete(r.refs, id)
 }
 
-// TODO: generate a safe reference id
-func (r *ExternalReferences) referenceid() int32 {
-	r.nextid++
-	return r.nextid
+// generates a random reference id that is not already in use.
+func (r *ExternalReferences) newReferenceId() int32 {
+	for {
+		id := int32(randomInt(1, 0x7FFFFFFF))
+		if _, ok := r.refs[id]; !ok {
+			return id
+		}
+	}
+}
+
+// Returns an int >= min, < max
+func randomInt(min, max int) int {
+	return min + rand.Intn(max-min)
 }