diff --git a/src/SlashCommands.js b/src/SlashCommands.js
index 344bac1ddb..711805f8ef 100644
--- a/src/SlashCommands.js
+++ b/src/SlashCommands.js
@@ -295,7 +295,7 @@ const commands = {
     // Define the power level of a user
     op: new Command("op", "<userId> [<power level>]", function(roomId, args) {
         if (args) {
-            const matches = args.match(/^(\S+?)( +(\d+))?$/);
+            const matches = args.match(/^(\S+?)( +(-?\d+))?$/);
             let powerLevel = 50; // default power level for op
             if (matches) {
                 const userId = matches[1];