diff --git a/src/chars.c b/src/chars.c
index de28d420..64a15e44 100644
--- a/src/chars.c
+++ b/src/chars.c
@@ -38,7 +38,7 @@
 #endif
 
 /* This function is equivalent to isblank(). */
-bool is_blank_char(unsigned char c)
+bool is_blank_char(unsigned int c)
 {
     return
 #ifdef HAVE_ISBLANK
@@ -86,9 +86,9 @@ bool is_blank_wchar(wchar_t wc)
 
 /* This function is equivalent to iscntrl(), except in that it also
  * handles control characters with their high bits set. */
-bool is_cntrl_char(unsigned char c)
+bool is_cntrl_char(unsigned int c)
 {
-    return (c < 32) || (127 <= c && c < 160);
+    return (0 <= c && c < 32) || (127 <= c && c < 160);
 }
 
 /* This function is equivalent to iscntrl() for multibyte characters,
diff --git a/src/proto.h b/src/proto.h
index 2e4c768e..6b6396da 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -151,12 +151,12 @@ extern char *homedir;
 /* Functions we want available. */
 
 /* Public functions in chars.c. */
-bool is_blank_char(unsigned char c);
+bool is_blank_char(unsigned int c);
 bool is_blank_mbchar(const char *c);
 #ifdef NANO_WIDE
 bool is_blank_wchar(wchar_t wc);
 #endif
-bool is_cntrl_char(unsigned char c);
+bool is_cntrl_char(unsigned int c);
 bool is_cntrl_mbchar(const char *c);
 #ifdef NANO_WIDE
 bool is_cntrl_wchar(wchar_t wc);