diff --git a/src/api/operator/numpy/random/np_multinomial_op.cc b/src/api/operator/numpy/random/np_multinomial_op.cc
index 3df4165bdca4..59ead4abe382 100644
--- a/src/api/operator/numpy/random/np_multinomial_op.cc
+++ b/src/api/operator/numpy/random/np_multinomial_op.cc
@@ -44,12 +44,12 @@ MXNET_REGISTER_API("_npi.multinomial")
   if (args[1].type_code() == kNull) {
     param.pvals = dmlc::nullopt;
   } else {
-    param.pvals = Tuple<double>(args[1].operator ObjectRef());
+    param.pvals = Obj2Tuple<double, Float>(args[1].operator ObjectRef());
   }
 
   // parse size
   if (args[2].type_code() == kNull) {
-    param.size = dmlc::nullopt;
+    param.size = dmlc::optional<mxnet::Tuple<int>>();
   } else {
     if (args[2].type_code() == kDLInt) {
       param.size = Tuple<int>(1, args[2].operator int64_t());