diff --git a/tools/fuzzing/libfuzzer/FuzzerDefs.h b/tools/fuzzing/libfuzzer/FuzzerDefs.h
--- a/tools/fuzzing/libfuzzer/FuzzerDefs.h
+++ b/tools/fuzzing/libfuzzer/FuzzerDefs.h
@@ -174,17 +174,17 @@ extern ExternalFunctions *EF;
 // We are using a custom allocator to give a different symbol name to STL
 // containers in order to avoid ODR violations.
 template<typename T>
   class fuzzer_allocator: public std::allocator<T> {
     public:
       fuzzer_allocator() = default;
 
       template<class U>
-      fuzzer_allocator(const fuzzer_allocator<U>&) {}
+      explicit fuzzer_allocator(const fuzzer_allocator<U>&) {}
 
       template<class Other>
       struct rebind { typedef fuzzer_allocator<Other> other;  };
   };
 
 template<typename T>
 using Vector = std::vector<T, fuzzer_allocator<T>>;