diff --git a/src/codegen.c b/src/codegen.c
index 2654fcbe3..8aef2a328 100644
--- a/src/codegen.c
+++ b/src/codegen.c
@@ -802,8 +802,9 @@ readint_float(codegen_scope *s, const char *p, int base)
   mrb_float f = 0;
   int n;
 
-  while (p <= e) {
-    char c = tolower(*p);
+  while (p < e) {
+    char c = *p;
+    c = tolower((unsigned char)c);
     for (n=0; n<base; n++) {
       if (mrb_digitmap[n] == c) {
 	f *= base;
diff --git a/src/parse.y b/src/parse.y
index 025707670..af91f09b6 100644
--- a/src/parse.y
+++ b/src/parse.y
@@ -3949,7 +3949,7 @@ parser_yylex(parser_state *p)
 	      }
 	      if (!ISXDIGIT(c)) break;
 	      nondigit = 0;
-	      tokadd(p, c);
+	      tokadd(p, tolower(c));
 	    } while ((c = nextc(p)) != -1);
 	  }
 	  pushback(p, c);