diff --git a/amalgamation/xgboost-all0.cc b/amalgamation/xgboost-all0.cc
index 6563daa44776..d3e0c23c616c 100644
--- a/amalgamation/xgboost-all0.cc
+++ b/amalgamation/xgboost-all0.cc
@@ -45,6 +45,7 @@
 
 // tress
 #include "../src/tree/split_evaluator.cc"
+#include "../src/tree/param.cc"
 #include "../src/tree/tree_model.cc"
 #include "../src/tree/tree_updater.cc"
 #include "../src/tree/updater_colmaker.cc"
diff --git a/src/tree/param.cc b/src/tree/param.cc
new file mode 100644
index 000000000000..8049501ea094
--- /dev/null
+++ b/src/tree/param.cc
@@ -0,0 +1,81 @@
+/*!
+ * Copyright by Contributors 2019
+ */
+#include <iostream>
+#include <vector>
+#include <utility>
+
+#include "param.h"
+
+namespace std {
+std::istream &operator>>(std::istream &is, std::vector<int> &t) {
+  t.clear();
+  // get (
+  while (true) {
+    char ch = is.peek();
+    if (isdigit(ch)) {
+      int idx;
+      if (is >> idx) {
+        t.emplace_back(idx);
+      }
+      return is;
+    }
+    is.get();
+    if (ch == '(') {
+      break;
+    }
+    if (!isspace(ch)) {
+      is.setstate(std::ios::failbit);
+      return is;
+    }
+  }
+  int idx;
+  std::vector<int> tmp;
+  while (true) {
+    char ch = is.peek();
+    if (isspace(ch)) {
+      is.get();
+    } else {
+      break;
+    }
+  }
+  if (is.peek() == ')') {
+    is.get();
+    return is;
+  }
+  while (is >> idx) {
+    tmp.push_back(idx);
+    char ch;
+    do {
+      ch = is.get();
+    } while (isspace(ch));
+    if (ch == 'L') {
+      ch = is.get();
+    }
+    if (ch == ',') {
+      while (true) {
+        ch = is.peek();
+        if (isspace(ch)) {
+          is.get();
+          continue;
+        }
+        if (ch == ')') {
+          is.get();
+          break;
+        }
+        break;
+      }
+      if (ch == ')') {
+        break;
+      }
+    } else if (ch == ')') {
+      break;
+    } else {
+      is.setstate(std::ios::failbit);
+      return is;
+    }
+  }
+  t = std::move(tmp);
+  return is;
+}
+}  // namespace std
diff --git a/src/tree/param.h b/src/tree/param.h
index ded36a313397..7c858c975284 100644
--- a/src/tree/param.h
+++ b/src/tree/param.h
@@ -495,63 +495,7 @@ inline std::ostream &operator<<(std::ostream &os, const std::vector<int> &t) {
   return os;
 }
 
-inline std::istream &operator>>(std::istream &is, std::vector<int> &t) {
-  // get (
-  while (true) {
-    char ch = is.peek();
-    if (isdigit(ch)) {
-      int idx;
-      if (is >> idx) {
-        t.assign(&idx, &idx + 1);
-      }
-      return is;
-    }
-    is.get();
-    if (ch == '(') {
-      break;
-    }
-    if (!isspace(ch)) {
-      is.setstate(std::ios::failbit);
-      return is;
-    }
-  }
-  int idx;
-  std::vector<int> tmp;
-  while (is >> idx) {
-    tmp.push_back(idx);
-    char ch;
-    do {
-      ch = is.get();
-    } while (isspace(ch));
-    if (ch == 'L') {
-      ch = is.get();
-    }
-    if (ch == ',') {
-      while (true) {
-        ch = is.peek();
-        if (isspace(ch)) {
-          is.get();
-          continue;
-        }
-        if (ch == ')') {
-          is.get();
-          break;
-        }
-        break;
-      }
-      if (ch == ')') {
-        break;
-      }
-    } else if (ch == ')') {
-      break;
-    } else {
-      is.setstate(std::ios::failbit);
-      return is;
-    }
-  }
-  t.assign(tmp.begin(), tmp.end());
-  return is;
-}
+std::istream &operator>>(std::istream &is, std::vector<int> &t);
 }  // namespace std
 
 #endif  // XGBOOST_TREE_PARAM_H_
diff --git a/tests/cpp/tree/test_param.cc b/tests/cpp/tree/test_param.cc
index 3f4e50ba245c..b4cc4005e3ad 100644
--- a/tests/cpp/tree/test_param.cc
+++ b/tests/cpp/tree/test_param.cc
@@ -74,6 +74,12 @@ TEST(Param, VectorStreamRead) {
   ss << "(3,2,1";
   ss >> vals_in;
   EXPECT_NE(vals_in, vals);
+
+  vals_in.clear(); ss.flush(); ss.clear(); ss.str("");
+  vals_in.emplace_back(3);
+  ss << "( )";
+  ss >> vals_in;
+  ASSERT_TRUE(ss.good());
 }
 
 TEST(Param, SplitEntry) {