diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1e96499..782f4d0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@ options. The project adheres to semantic versioning.
 ### Bug Fixes
 
 * Fixed version number in header.
+* Fixed cast warning in Unicode function.
 
 ## 3.1
 
diff --git a/include/cxxopts.hpp b/include/cxxopts.hpp
index 6721342..2d3f4e8 100644
--- a/include/cxxopts.hpp
+++ b/include/cxxopts.hpp
@@ -230,10 +230,10 @@ stringAppend(String& s, Iterator begin, Iterator end)
 }
 
 inline
-std::size_t
+size_t
 stringLength(const String& s)
 {
-  return s.length();
+  return static_cast<size_t>(s.length());
 }
 
 inline