diff --git a/libs/binder/include/binder/IInterface.h b/libs/binder/include/binder/IInterface.h
index 79d9b79..cabfc7f 100644
--- a/libs/binder/include/binder/IInterface.h
+++ b/libs/binder/include/binder/IInterface.h
@@ -20,6 +20,8 @@
 
 #include <binder/Binder.h>
 
+#include <assert.h>
+
 namespace android {
 
 // ----------------------------------------------------------------------
@@ -155,7 +157,11 @@
     std::unique_ptr<I##INTERFACE> I##INTERFACE::default_impl;           \
     bool I##INTERFACE::setDefaultImpl(std::unique_ptr<I##INTERFACE> impl)\
     {                                                                   \
-        if (!I##INTERFACE::default_impl && impl) {                      \
+        /* Only one user of this interface can use this function     */ \
+        /* at a time. This is a heuristic to detect if two different */ \
+        /* users in the same process use this function.              */ \
+        assert(!I##INTERFACE::default_impl);                            \
+        if (impl) {                                                     \
             I##INTERFACE::default_impl = std::move(impl);               \
             return true;                                                \
         }                                                               \