diff --git a/NEWS b/NEWS
index ee63bae14d6..01ce2fcb14e 100644
--- a/NEWS
+++ b/NEWS
@@ -62,6 +62,10 @@ PHP                                                                        NEWS
   . Removed the deprecated inet_ntoa call support. (David Carlier)
   . Fixed bug #63937 (Upload speed 10 times slower with PHP). (nielsdos)
 
+- GD:
+  . Fix parameter numbers and missing alpha check for imagecolorset().
+    (Giovanni Giacobbi)
+
 - Gettext:
   . bind_textdomain_codeset, textdomain and d(*)gettext functions
     now throw an exception on empty domain. (David Carlier)
diff --git a/ext/gd/gd.c b/ext/gd/gd.c
index 8b8d6cfd848..b04bd18bb92 100644
--- a/ext/gd/gd.c
+++ b/ext/gd/gd.c
@@ -2212,9 +2212,10 @@ PHP_FUNCTION(imagecolorset)
 
 	im = php_gd_libgdimageptr_from_zval_p(IM);
 
-	CHECK_RGBA_RANGE(red, Red, 2);
-	CHECK_RGBA_RANGE(green, Green, 3);
-	CHECK_RGBA_RANGE(blue, Blue, 4);
+	CHECK_RGBA_RANGE(red, Red, 3);
+	CHECK_RGBA_RANGE(green, Green, 4);
+	CHECK_RGBA_RANGE(blue, Blue, 5);
+	CHECK_RGBA_RANGE(alpha, Alpha, 6);
 
 	col = color;
 
diff --git a/ext/gd/tests/imagecolorset_error1.phpt b/ext/gd/tests/imagecolorset_error1.phpt
new file mode 100644
index 00000000000..0a529f8925e
--- /dev/null
+++ b/ext/gd/tests/imagecolorset_error1.phpt
@@ -0,0 +1,26 @@
+--TEST--
+imagecolorset() parameters errors
+--EXTENSIONS--
+gd
+--FILE--
+<?php
+
+require __DIR__ . '/func.inc';
+
+$im = imagecreate(5, 5);
+
+$c = imagecolorallocatealpha($im, 3, 4, 5, 6);
+
+trycatch_dump(
+    fn() => imagecolorset($im, $c, -3, 4, 5, 6),
+    fn() => imagecolorset($im, $c, 3, -4, 5, 6),
+    fn() => imagecolorset($im, $c, 3, 4, -5, 6),
+    fn() => imagecolorset($im, $c, 3, 4, 5, -6),
+);
+
+?>
+--EXPECT--
+!! [ValueError] imagecolorset(): Argument #3 ($red) must be between 0 and 255 (inclusive)
+!! [ValueError] imagecolorset(): Argument #4 ($green) must be between 0 and 255 (inclusive)
+!! [ValueError] imagecolorset(): Argument #5 ($blue) must be between 0 and 255 (inclusive)
+!! [ValueError] imagecolorset(): Argument #6 ($alpha) must be between 0 and 127 (inclusive)