diff --git a/desktop_version/src/UtilityClass.cpp b/desktop_version/src/UtilityClass.cpp
index 06859db2..ca5b9c47 100644
--- a/desktop_version/src/UtilityClass.cpp
+++ b/desktop_version/src/UtilityClass.cpp
@@ -213,7 +213,7 @@ bool is_positive_num(const std::string& str)
 {
 	for (size_t i = 0; i < str.length(); i++)
 	{
-		if (!std::isdigit(str[i]))
+		if (!std::isdigit(static_cast<unsigned char>(str[i])))
 		{
 			return false;
 		}