diff --git a/Zend/tests/fibers/negative_stack_size.phpt b/Zend/tests/fibers/negative_stack_size.phpt
new file mode 100644
index 00000000000..591a8f28878
--- /dev/null
+++ b/Zend/tests/fibers/negative_stack_size.phpt
@@ -0,0 +1,16 @@
+--TEST--
+fiber.stack_size must be a positive number
+--FILE--
+<?php
+ini_set("fiber.stack_size","-1");
+$fiber = new Fiber(function() {});
+try {
+    $fiber->start();
+} catch (Throwable $e) {
+	echo "Exception: " . $e->getMessage()."\n";
+}
+?>
+DONE
+--EXPECTF--
+Warning: fiber.stack_size must be a positive number in %snegative_stack_size.php on line 2
+DONE
diff --git a/Zend/zend.c b/Zend/zend.c
index 46da1ebd21e..fb0af12f9e5 100644
--- a/Zend/zend.c
+++ b/Zend/zend.c
@@ -230,7 +230,12 @@ static ZEND_INI_MH(OnUpdateReservedStackSize) /* {{{ */
 static ZEND_INI_MH(OnUpdateFiberStackSize) /* {{{ */
 {
 	if (new_value) {
-		EG(fiber_stack_size) = zend_ini_parse_uquantity_warn(new_value, entry->name);
+		zend_long tmp = zend_ini_parse_quantity_warn(new_value, entry->name);
+		if (tmp < 0) {
+			zend_error(E_WARNING, "fiber.stack_size must be a positive number");
+			return FAILURE;
+		}
+		EG(fiber_stack_size) = tmp;
 	} else {
 		EG(fiber_stack_size) = ZEND_FIBER_DEFAULT_C_STACK_SIZE;
 	}