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-rw-r--r--src/option.cc3
1 files changed, 2 insertions, 1 deletions
diff --git a/src/option.cc b/src/option.cc
index 43dd853c..44b135e6 100644
--- a/src/option.cc
+++ b/src/option.cc
@@ -132,7 +132,8 @@ void process_environment(const char ** envp, const string& tag,
if (*q == '_')
*r++ = '-';
else
- *r++ = static_cast<char>(std::tolower(*q));
+ *r++ = static_cast<char>(std::tolower(
+ static_cast<unsigned char>(*q)));
*r = '\0';
if (*q == '=') {