diff --git a/keystore/blob.cpp b/keystore/blob.cpp
index f887e80..a636065 100644
--- a/keystore/blob.cpp
+++ b/keystore/blob.cpp
@@ -139,7 +139,7 @@
 
 class ArrayStreamBuffer : public std::streambuf {
   public:
-    template <typename T, size_t size> ArrayStreamBuffer(const T (&data)[size]) {
+    template <typename T, size_t size> explicit ArrayStreamBuffer(const T (&data)[size]) {
         static_assert(sizeof(T) == 1, "Array element size too large");
         std::streambuf::char_type* d = const_cast<std::streambuf::char_type*>(
             reinterpret_cast<const std::streambuf::char_type*>(&data[0]));