diff --git a/ext/mbstring/mbstring.c b/ext/mbstring/mbstring.c
index c5331ef017af6..ae923469ec328 100644
--- a/ext/mbstring/mbstring.c
+++ b/ext/mbstring/mbstring.c
@@ -5856,6 +5856,11 @@ PHP_FUNCTION(mb_str_pad)
 	}
 
 	size_t pad_length = mb_get_strlen(pad, encoding);
+	if (pad_length == 0) {
+		/* Possible with invalidly encoded padding string. */
+		zend_argument_must_not_be_empty_error(3);
+		RETURN_THROWS();
+	}
 
 	size_t num_mb_pad_chars = pad_to_length - input_length;
 
diff --git a/ext/mbstring/tests/gh20833.phpt b/ext/mbstring/tests/gh20833.phpt
new file mode 100644
index 0000000000000..099aa3379238f
--- /dev/null
+++ b/ext/mbstring/tests/gh20833.phpt
@@ -0,0 +1,16 @@
+--TEST--
+GH-20833 (mb_str_pad() divide by zero if padding string is invalid in the encoding)
+--EXTENSIONS--
+mbstring
+--FILE--
+<?php
+$utf8 = "test";
+$utf32 = mb_convert_encoding($utf8, 'UTF-32', 'UTF-8');
+try {
+    mb_str_pad($utf32, 5, "1" /* invalid for encoding */, STR_PAD_RIGHT, "UTF-32");
+} catch (ValueError $e) {
+    echo $e::class, ": ", $e->getMessage(), "\n";
+}
+?>
+--EXPECT--
+ValueError: mb_str_pad(): Argument #3 ($pad_string) must not be empty