diff --git a/ext/gmp/gmp.c b/ext/gmp/gmp.c
index cf6580e95e9b8..a209745809044 100644
--- a/ext/gmp/gmp.c
+++ b/ext/gmp/gmp.c
@@ -1092,8 +1092,10 @@ ZEND_FUNCTION(gmp_fact)
 		RETURN_THROWS();
 	}
 
-	// TODO: Check that we don't an int that is larger than an unsigned long?
-	// Could use mpz_fits_slong_p() if we revert to using mpz_get_si()
+	if (!mpz_fits_ulong_p(gmpnum)) {
+		zend_argument_value_error(1, "must be between 0 and %lu", ULONG_MAX);
+		RETURN_THROWS();
+	}
 
 	INIT_GMP_RETVAL(gmpnum_result);
 	mpz_fac_ui(gmpnum_result, mpz_get_ui(gmpnum));
diff --git a/ext/gmp/tests/gmp_fact_overflow.phpt b/ext/gmp/tests/gmp_fact_overflow.phpt
new file mode 100644
index 0000000000000..2d22005818c50
--- /dev/null
+++ b/ext/gmp/tests/gmp_fact_overflow.phpt
@@ -0,0 +1,25 @@
+--TEST--
+gmp_fact() rejects values larger than unsigned long
+--EXTENSIONS--
+gmp
+--FILE--
+<?php
+
+try {
+    var_dump(gmp_fact(gmp_pow(2, 100)));
+} catch (\ValueError $e) {
+    echo $e->getMessage() . \PHP_EOL;
+}
+
+try {
+    var_dump(gmp_fact(gmp_init("18446744073709551616")));
+} catch (\ValueError $e) {
+    echo $e->getMessage() . \PHP_EOL;
+}
+
+echo "Done\n";
+?>
+--EXPECTF--
+gmp_fact(): Argument #1 ($num) must be between 0 and %d
+gmp_fact(): Argument #1 ($num) must be between 0 and %d
+Done