diff --git a/src/Seq.Api/Model/Signals/SignalExpressionPart.cs b/src/Seq.Api/Model/Signals/SignalExpressionPart.cs
index c0615e3..c5bda8f 100644
--- a/src/Seq.Api/Model/Signals/SignalExpressionPart.cs
+++ b/src/Seq.Api/Model/Signals/SignalExpressionPart.cs
@@ -74,7 +74,7 @@ public override string ToString()
                 return $"{Group(Left)},{Group(Right)}";
 
             if (Kind == SignalExpressionKind.Union)
-                return $"{Group(Left)}+{Group(Right)}";
+                return $"{Group(Left)}~{Group(Right)}";
 
             throw new InvalidOperationException("Invalid signal expression kind.");
         }