diff --git a/src/hlua.c b/src/hlua.c
index 1c87daae3..be959d847 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -420,6 +420,9 @@ static int hlua_lua2arg(lua_State *L, int ud, struct arg *arg)
 	case LUA_TSTRING:
 		arg->type = ARGT_STR;
 		arg->data.str.area = (char *)lua_tolstring(L, ud, (size_t *)&arg->data.str.data);
+		/* We don't know the actual size of the underlying allocation, so be conservative. */
+		arg->data.str.size = arg->data.str.data;
+		arg->data.str.head = 0;
 		break;
 
 	case LUA_TUSERDATA:
@@ -560,6 +563,9 @@ static int hlua_lua2smp(lua_State *L, int ud, struct sample *smp)
 		smp->data.type = SMP_T_STR;
 		smp->flags |= SMP_F_CONST;
 		smp->data.u.str.area = (char *)lua_tolstring(L, ud, (size_t *)&smp->data.u.str.data);
+		/* We don't know the actual size of the underlying allocation, so be conservative. */
+		smp->data.u.str.size = smp->data.u.str.data;
+		smp->data.u.str.head = 0;
 		break;
 
 	case LUA_TUSERDATA: