diff --git a/indra/llcommon/llsdserialize.cpp b/indra/llcommon/llsdserialize.cpp
index a14a6b5b1bc33e8fe2d45f595f73ccb6bcee2c47..99d0c39694c90bc1b5e330d2b36a34e4dc9ab35c 100644
--- a/indra/llcommon/llsdserialize.cpp
+++ b/indra/llcommon/llsdserialize.cpp
@@ -143,7 +143,7 @@ bool LLSDSerialize::deserialize(LLSD& sd, std::istream& str, llssize max_bytes)
 		// byte. We could store one if needed, since even the incremented
 		// inbuf won't exceed sizeof(hdr_buf)-1, but there's no need.
 	}
-	std::string header{ hdr_buf, inbuf };
+	std::string header{ hdr_buf, narrow(inbuf) };
 	if (str.fail())
 	{
 		str.clear();