diff --git a/lib/univalue_utffilter.h b/lib/univalue_utffilter.h
index b4a9ddc0a31003..20d40430095841 100644
--- a/lib/univalue_utffilter.h
+++ b/lib/univalue_utffilter.h
@@ -13,7 +13,7 @@
 class JSONUTF8StringFilter
 {
 public:
-    JSONUTF8StringFilter(std::string &s):
+    explicit JSONUTF8StringFilter(std::string &s):
         str(s), is_valid(true), codepoint(0), state(0), surpair(0)
     {
     }