diff --git a/include/WinStd/Common.h b/include/WinStd/Common.h
index d94f174a..08202d57 100644
--- a/include/WinStd/Common.h
+++ b/include/WinStd/Common.h
@@ -277,7 +277,7 @@ static int vsprintf(_Inout_ std::basic_string<char, _Traits, _Ax> &str, _In_z_ _
     }
     size_t offset = str.size();
     str.resize(offset + count);
-    if (_vsnprintf(&str[offset], count + 1, format, arg) != count)
+    if (_vsnprintf(&str[offset], static_cast<size_t>(count) + 1, format, arg) != count)
         throw std::runtime_error("failed to format string");
     return count;
 }
@@ -316,7 +316,7 @@ static int vsprintf(_Inout_ std::basic_string<wchar_t, _Traits, _Ax> &str, _In_z
     }
     size_t offset = str.size();
     str.resize(offset + count);
-    if (_vsnwprintf(&str[offset], count + 1, format, arg) != count)
+    if (_vsnwprintf(&str[offset], static_cast<size_t>(count) + 1, format, arg) != count)
         throw std::runtime_error("failed to format string");
     return count;
 }