diff --git a/common/text/tree_utils.h b/common/text/tree_utils.h
index 9f6a42a81..7300de524 100644
--- a/common/text/tree_utils.h
+++ b/common/text/tree_utils.h
@@ -17,6 +17,7 @@
 #ifndef VERIBLE_COMMON_TEXT_TREE_UTILS_H_
 #define VERIBLE_COMMON_TEXT_TREE_UTILS_H_
 
+#include <cstddef>
 #include <functional>
 #include <iosfwd>
 
@@ -112,7 +113,8 @@ const SyntaxTreeNode* CheckOptionalSymbolAsNode(const SPtr& symbol,
 
 // Specialization for nullptr_t.
 template <typename E>
-const SyntaxTreeNode* CheckOptionalSymbolAsNode(const nullptr_t& symbol, E) {
+const SyntaxTreeNode* CheckOptionalSymbolAsNode(const std::nullptr_t& symbol,
+                                                E) {
   return nullptr;
 }
 
@@ -127,7 +129,8 @@ const SyntaxTreeLeaf* CheckOptionalSymbolAsLeaf(const SPtr& symbol,
 
 // Specialization for nullptr_t.
 template <typename E>
-const SyntaxTreeLeaf* CheckOptionalSymbolAsLeaf(const nullptr_t& symbol, E) {
+const SyntaxTreeLeaf* CheckOptionalSymbolAsLeaf(const std::nullptr_t& symbol,
+                                                E) {
   return nullptr;
 }