diff --git a/libc/src/string/strspn.cpp b/libc/src/string/strspn.cpp
index 60f16df8acb1e..9580d1bc403e8 100644
--- a/libc/src/string/strspn.cpp
+++ b/libc/src/string/strspn.cpp
@@ -19,8 +19,9 @@ LLVM_LIBC_FUNCTION(size_t, strspn, (const char *src, const char *segment)) {
   cpp::bitset<256> bitset;
 
   for (; *segment; ++segment)
-    bitset.set(*segment);
-  for (; *src && bitset.test(*src); ++src)
+    bitset.set(*reinterpret_cast<const unsigned char *>(segment));
+  for (; *src && bitset.test(*reinterpret_cast<const unsigned char *>(src));
+       ++src)
     ;
   return src - initial;
 }