diff --git a/CMakeLists.txt b/CMakeLists.txt
index 613f0ab3..723e2344 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -138,7 +138,6 @@ if (NOT MSVC)
     check_function_exists(ftime HAVE_FTIME)
     check_function_exists(getentropy HAVE_GETENTROPY)
     check_function_exists(gettimeofday HAVE_GETTIMEOFDAY)
-    check_function_exists(isascii HAVE_ISASCII)
     check_library_exists(history append_history "" HAVE_LIBHISTORY)
     check_library_exists(readline readline "" HAVE_LIBREADLINE)
     check_function_exists(mmap HAVE_MMAP)
diff --git a/config.h.cmake.in b/config.h.cmake.in
index e28b6258..2f4aeba3 100644
--- a/config.h.cmake.in
+++ b/config.h.cmake.in
@@ -28,9 +28,6 @@
 /* Define to 1 if you have the `gettimeofday' function. */
 #cmakedefine HAVE_GETTIMEOFDAY 1
 
-/* Define to 1 if you have the `isascii' function. */
-#cmakedefine HAVE_ISASCII 1
-
 /* Define if history library is there (-lhistory) */
 #cmakedefine HAVE_LIBHISTORY 1
 
diff --git a/configure.ac b/configure.ac
index 4a594b52..270e3635 100644
--- a/configure.ac
+++ b/configure.ac
@@ -313,7 +313,7 @@ AC_CHECK_HEADERS([glob.h])
 AM_CONDITIONAL(WITH_GLOB, test "$ac_cv_header_glob_h" = "yes")
 
 dnl Checks for library functions.
-AC_CHECK_FUNCS([getentropy gettimeofday ftime isascii stat mmap munmap])
+AC_CHECK_FUNCS([getentropy gettimeofday ftime stat mmap munmap])
 
 AH_VERBATIM([HAVE_MUNMAP_AFTER],[/* mmap() is no good without munmap() */
 #if defined(HAVE_MMAP) && !defined(HAVE_MUNMAP)
diff --git a/meson.build b/meson.build
index c9534d80..d0d631d9 100644
--- a/meson.build
+++ b/meson.build
@@ -331,7 +331,6 @@ xml_check_functions = [
     ['gettimeofday', 'sys/time.h'],
     ['ftime', 'sys/timeb.h'],
     ['stat', 'sys/stat.h'],
-    ['isascii', 'ctype.h'],
     ['mmap', 'sys/mman.h'],
     ['munmap', 'sys/mman.h'],
 ]