diff --git a/src/util/zstring.cpp b/src/util/zstring.cpp
index 570510458..eaa5bb5ee 100644
--- a/src/util/zstring.cpp
+++ b/src/util/zstring.cpp
@@ -78,7 +78,7 @@ zstring::zstring(char const* s) {
             m_buffer.push_back(ch);
         }
         else {
-            m_buffer.push_back(*s);
+            m_buffer.push_back((unsigned char)*s);
             ++s;
         }
     }