diff --git a/libwvdrmengine/cdm/metrics/include/event_metric.h b/libwvdrmengine/cdm/metrics/include/event_metric.h
index 61012fa2..4e81ad62 100644
--- a/libwvdrmengine/cdm/metrics/include/event_metric.h
+++ b/libwvdrmengine/cdm/metrics/include/event_metric.h
@@ -238,20 +238,57 @@ inline void AppendFieldNames(std::vector<std::string>* field_name_vector,
   va_end(field_names);
 }
 
+// These helper methods and FirstUnusedType assure that there is no mismatch
+// between the specified types for EventMetrics and the constructors and
+// methods used for the specializations.
+template <bool>
+struct CompileAssert {};
+#define COMPILE_ASSERT(expr, msg) \
+  typedef impl::CompileAssert<(bool(expr))> msg[bool(expr) ? 1 : -1]
+
+template <typename T> struct is_unused { static const bool value = false; };
+template <> struct is_unused<Unused> { static const bool value = true; };
+
+template <typename F1, typename F2, typename F3, typename F4>
+class FirstUnusedType {
+  static const bool a = is_unused<F1>::value;
+  static const bool b = is_unused<F2>::value;
+  static const bool c = is_unused<F3>::value;
+  static const bool d = is_unused<F4>::value;
+  // Check that all types after the first Unused are also Unused.
+  COMPILE_ASSERT(a <= b, Invalid_Unused_At_Position_2);
+  COMPILE_ASSERT(b <= c, Invalid_Unused_At_Position_3);
+  COMPILE_ASSERT(c <= d, Invalid_Unused_At_Position_4);
+
+ public:
+  static const int value = 5 - (a + b + c + d);
+};
+
+// Asserts that no Unused types exist before N; after N, are all Unused types.
+#define ASSERT_METRIC_UNUSED_START_FROM(N) \
+  COMPILE_ASSERT((\
+      impl::FirstUnusedType<F1, F2, F3, F4>::value) == N, \
+          Unused_Start_From_##N)
+
 }  // namespace impl
 
 // Overloaded template constructor implementations for EventMetric.
 template<typename F1, typename F2, typename F3, typename F4>
 EventMetric<F1, F2, F3, F4>::EventMetric(
     const std::string& metric_name)
-        : BaseEventMetric(metric_name) {}
+        : BaseEventMetric(metric_name) {
+  ASSERT_METRIC_UNUSED_START_FROM(1);
+}
 
 template<typename F1, typename F2, typename F3, typename F4>
 EventMetric<F1, F2, F3, F4>::EventMetric(
     const std::string& metric_name,
     const char* field_name)
         : BaseEventMetric(metric_name) {
-  impl::AppendFieldNames(&field_names_, 1, field_name);
+  ASSERT_METRIC_UNUSED_START_FROM(2);
+  impl::AppendFieldNames(&field_names_,
+                         impl::FirstUnusedType<F1, F2, F3, F4>::value - 1,
+                         field_name);
 }
 template<typename F1, typename F2, typename F3, typename F4>
 EventMetric<F1, F2, F3, F4>::EventMetric(
@@ -259,7 +296,10 @@ EventMetric<F1, F2, F3, F4>::EventMetric(
     const char* field_name1,
     const char* field_name2)
         : BaseEventMetric(metric_name) {
-  impl::AppendFieldNames(&field_names_, 2, field_name1, field_name2);
+  ASSERT_METRIC_UNUSED_START_FROM(3);
+  impl::AppendFieldNames(&field_names_,
+                         impl::FirstUnusedType<F1, F2, F3, F4>::value - 1,
+                         field_name1, field_name2);
 }
 template<typename F1, typename F2, typename F3, typename F4>
 EventMetric<F1, F2, F3, F4>::EventMetric(
@@ -268,8 +308,10 @@ EventMetric<F1, F2, F3, F4>::EventMetric(
     const char* field_name2,
     const char* field_name3)
         : BaseEventMetric(metric_name) {
+  ASSERT_METRIC_UNUSED_START_FROM(4);
   impl::AppendFieldNames(&field_names_,
-                         3, field_name1, field_name2, field_name3);
+                         impl::FirstUnusedType<F1, F2, F3, F4>::value - 1,
+                         field_name1, field_name2, field_name3);
 }
 template<typename F1, typename F2, typename F3, typename F4>
 EventMetric<F1, F2, F3, F4>::EventMetric(
@@ -279,8 +321,10 @@ EventMetric<F1, F2, F3, F4>::EventMetric(
     const char* field_name3,
     const char* field_name4)
         : BaseEventMetric(metric_name) {
+  ASSERT_METRIC_UNUSED_START_FROM(5);
   impl::AppendFieldNames(&field_names_,
-                         4, field_name1, field_name2,
+                         impl::FirstUnusedType<F1, F2, F3, F4>::value - 1,
+                         field_name1, field_name2,
                          field_name3, field_name4);
 }