diff --git a/double-conversion/utils.h b/double-conversion/utils.h
index 41078b6c..868770d2 100644
--- a/double-conversion/utils.h
+++ b/double-conversion/utils.h
@@ -326,7 +326,7 @@ class StringBuilder {
   void AddSubstring(const char* s, int n) {
     DOUBLE_CONVERSION_ASSERT(!is_finalized() && position_ + n < buffer_.length());
     DOUBLE_CONVERSION_ASSERT(static_cast<size_t>(n) <= strlen(s));
-    memmove(&buffer_[position_], s, n);
+    memmove(&buffer_[position_], s, static_cast<size_t>(n));
     position_ += n;
   }