diff --git a/configure.ac b/configure.ac
index 6feb5de5..3807b4cb 100644
--- a/configure.ac
+++ b/configure.ac
@@ -101,7 +101,6 @@ AC_CHECK_FUNC([fcntl],,    AC_MSG_ERROR("Could not find fcntl"))
 AC_CHECK_FUNC([setlocale],,AC_MSG_ERROR("Could not find setlocale"))
 AC_CHECK_FUNC([atexit],,   AC_MSG_ERROR("Could not find atexit in c library"))
 AC_CHECK_FUNC([glob],,     AC_MSG_ERROR("Could not find glob in c library"))
-AC_CHECK_FUNC([toupper],,  AC_MSG_ERROR("Could not find toupper in c library"))
 
 AC_CHECK_HEADER([math.h],, AC_MSG_ERROR("Could not find math.h header file"))
 AC_SEARCH_LIBS([floor],[m],,      AC_MSG_ERROR("Could not find floor in math library"))
diff --git a/lexer/theme-lexer.l b/lexer/theme-lexer.l
index 235a3f7e..a5cc1ed1 100644
--- a/lexer/theme-lexer.l
+++ b/lexer/theme-lexer.l
@@ -37,7 +37,6 @@
 #include <gio/gio.h>
 #include <helper.h>
 #include <math.h>
-#include <ctype.h>
 #include <strings.h>
 #include "rofi.h"
 #include "theme.h"
@@ -92,10 +91,10 @@ static double rofi_theme_parse_convert_hex ( char high, char low)
 {
     uint8_t retv = 0;
 
-    int t = toupper(high);
+    int t = g_ascii_toupper ( high );
     t = ( t > '9')? (t-'A'+10):(t-'0');
     retv = t<<4;
-    t = toupper ( low );
+    t = g_ascii_toupper ( low );
     t = ( t > '9')? (t-'A'+10):(t-'0');
     retv +=t;
     return retv/255.0;