diff --git external/libucl/src/ucl_internal.h external/libucl/src/ucl_internal.h
index a6d8e09..36e86b8 100644
--- external/libucl/src/ucl_internal.h
+++ external/libucl/src/ucl_internal.h
@@ -200,33 +200,33 @@ ucl_maybe_parse_boolean (ucl_object_t *obj, const unsigned char *start, size_t l
 	bool ret = false, val = false;
 
 	if (len == 5) {
-		if (tolower (p[0]) == 'f' && strncasecmp (p, "false", 5) == 0) {
+		if ((p[0] == 'f' || p[0] == 'F') && strncasecmp (p, "false", 5) == 0) {
 			ret = true;
 			val = false;
 		}
 	}
 	else if (len == 4) {
-		if (tolower (p[0]) == 't' && strncasecmp (p, "true", 4) == 0) {
+		if ((p[0] == 't' || p[0] == 'T') && strncasecmp (p, "true", 4) == 0) {
 			ret = true;
 			val = true;
 		}
 	}
 	else if (len == 3) {
-		if (tolower (p[0]) == 'y' && strncasecmp (p, "yes", 3) == 0) {
+		if ((p[0] == 'y' || p[0] == 'Y') && strncasecmp (p, "yes", 3) == 0) {
 			ret = true;
 			val = true;
 		}
-		if (tolower (p[0]) == 'o' && strncasecmp (p, "off", 3) == 0) {
+		else if ((p[0] == 'o' || p[0] == 'O') && strncasecmp (p, "off", 3) == 0) {
 			ret = true;
 			val = false;
 		}
 	}
 	else if (len == 2) {
-		if (tolower (p[0]) == 'n' && strncasecmp (p, "no", 2) == 0) {
+		if ((p[0] == 'n' || p[0] == 'N') && strncasecmp (p, "no", 2) == 0) {
 			ret = true;
 			val = false;
 		}
-		else if (tolower (p[0]) == 'o' && strncasecmp (p, "on", 2) == 0) {
+		else if ((p[0] == 'o' || p[0] == 'O') && strncasecmp (p, "on", 2) == 0) {
 			ret = true;
 			val = true;
 		}