diff --git a/src/chars.c b/src/chars.c
index c9f82c89..ead9da67 100644
--- a/src/chars.c
+++ b/src/chars.c
@@ -41,7 +41,7 @@
 
 #ifndef HAVE_ISBLANK
 /* This function is equivalent to isblank(). */
-int nisblank(int c)
+bool nisblank(int c)
 {
     return isspace(c) && (c == '\t' || !is_cntrl_char(c));
 }
@@ -49,7 +49,7 @@ int nisblank(int c)
 
 #if !defined(HAVE_ISWBLANK) && defined(NANO_WIDE)
 /* This function is equivalent to iswblank(). */
-int niswblank(wchar_t wc)
+bool niswblank(wchar_t wc)
 {
     return iswspace(wc) && (wc == '\t' || !is_cntrl_wchar(wc));
 }
diff --git a/src/proto.h b/src/proto.h
index 3ec20383..f584d46a 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -160,10 +160,10 @@ extern char *homedir;
 
 /* Public functions in chars.c. */
 #ifndef HAVE_ISBLANK
-int nisblank(int c);
+bool nisblank(int c);
 #endif
 #if !defined(HAVE_ISWBLANK) && defined(NANO_WIDE)
-int niswblank(wchar_t wc);
+bool niswblank(wchar_t wc);
 #endif
 bool is_byte(int c);
 bool is_alnum_mbchar(const char *c);