Index: libc/src/string/string_utils.h
===================================================================
--- libc/src/string/string_utils.h
+++ libc/src/string/string_utils.h
@@ -166,8 +166,8 @@
   cpp::bitset<256> bitset;
 
   for (; *segment; ++segment)
-    bitset.set(*segment);
-  for (; *src && !bitset.test(*src); ++src)
+    bitset.set((uint8_t)*segment);
+  for (; *src && !bitset.test((uint8_t)*src); ++src)
     ;
   return src - initial;
 }