--- a/js/src/gc/StoreBuffer.h
+++ b/js/src/gc/StoreBuffer.h
@@ -444,17 +444,17 @@ class ArenaCellSet
 
     // Pointer to next set forming a linked list.
     ArenaCellSet* next;
 
     // Bit vector for each possible cell start position.
     BitArray<ArenaCellCount> bits;
 
   public:
-    ArenaCellSet(Arena* arena);
+    explicit ArenaCellSet(Arena* arena);
 
     bool hasCell(const TenuredCell* cell) const {
         return hasCell(getCellIndex(cell));
     }
 
     void putCell(const TenuredCell* cell) {
         putCell(getCellIndex(cell));
     }