diff --git a/runtime/gc_map.h b/runtime/gc_map.h
index 33d09f2..ffe54c4 100644
--- a/runtime/gc_map.h
+++ b/runtime/gc_map.h
@@ -27,7 +27,7 @@
 // Lightweight wrapper for native PC offset to reference bit maps.
 class NativePcOffsetToReferenceMap {
  public:
-  NativePcOffsetToReferenceMap(const uint8_t* data) : data_(data) {
+  explicit NativePcOffsetToReferenceMap(const uint8_t* data) : data_(data) {
     CHECK(data_ != NULL);
   }