diff --git a/PluralKit.API/Startup.cs b/PluralKit.API/Startup.cs
index f14dafba..67626ffa 100644
--- a/PluralKit.API/Startup.cs
+++ b/PluralKit.API/Startup.cs
@@ -133,6 +133,15 @@ namespace PluralKit.API
             app.UseExceptionHandler(handler => handler.Run(async ctx =>
             {
                 var exc = ctx.Features.Get<IExceptionHandlerPathFeature>();
+
+                // handle common ISEs that are generated by invalid user input
+                if (exc.Error is InvalidCastException && exc.Error.Message.Contains("Newtonsoft.Json"))
+                {
+                    ctx.Response.StatusCode = 400;
+                    await ctx.Response.WriteAsync("{\"message\":\"400: Bad Request\",\"code\":0}");
+                    return;
+                }
+
                 if (exc.Error is not PKError)
                 {
                     ctx.Response.StatusCode = 500;