diff --git a/lib/System/Path.cpp b/lib/System/Path.cpp
index 4c965db..72bd7ad 100644
--- a/lib/System/Path.cpp
+++ b/lib/System/Path.cpp
@@ -55,7 +55,7 @@
 sys::IdentifyFileType(const char *magic, unsigned length) {
   assert(magic && "Invalid magic number string");
   assert(length >=4 && "Invalid magic number length");
-  switch (magic[0]) {
+  switch ((unsigned char)magic[0]) {
     case 0xDE:  // 0x0B17C0DE = BC wraper
       if (magic[1] == (char)0xC0 && magic[2] == (char)0x17 &&
           magic[3] == (char)0x0B)