--- chicken-4.11.0rc1/runtime.c
+++ chicken-4.11.0rc1.patched/runtime.c
@@ -9236,7 +9236,7 @@
       return (C_word)(*(*str - 1));
 
     case C_FIXNUM_BIT:
-      val = (C_uword)*((*str)++) << 24; /* always big endian */
+      val = (C_uword)(signed char)*((*str)++) << 24; /* always big endian */
       val |= ((C_uword)*((*str)++) & 0xff) << 16;
       val |= ((C_uword)*((*str)++) & 0xff) << 8;
       val |= ((C_uword)*((*str)++) & 0xff);