diff --git a/src/runtime/js.zig b/src/runtime/js.zig
index 2eb5e1bc..357ce34c 100644
--- a/src/runtime/js.zig
+++ b/src/runtime/js.zig
@@ -274,12 +274,20 @@ pub fn Env(comptime State: type, comptime WebApis: type) type {
         }
 
         pub fn pumpMessageLoop(self: *const Self) bool {
-            if (self.platform == null) return false;
+            if (self.platform == null) {
+                // In test mode only, platform can be null.
+                if (builtin.is_test) return false;
+                @panic("platform is null");
+            }
             return self.platform.?.inner.pumpMessageLoop(self.isolate, false);
         }
 
         pub fn runIdleTasks(self: *const Self) void {
-            if (self.platform == null) return;
+            if (self.platform == null) {
+                // In test mode only, platform can be null.
+                if (builtin.is_test) return;
+                @panic("platform is null");
+            }
             return self.platform.?.inner.runIdleTasks(self.isolate, 1);
         }
 
diff --git a/src/runtime/testing.zig b/src/runtime/testing.zig
index 4ecd194f..25695232 100644
--- a/src/runtime/testing.zig
+++ b/src/runtime/testing.zig
@@ -42,7 +42,7 @@ pub fn Runner(comptime State: type, comptime Global: type, comptime types: anyty
             const self = try allocator.create(Self);
             errdefer allocator.destroy(self);
 
-            self.env = try Env.init(allocator, .{});
+            self.env = try Env.init(allocator, null, .{});
             errdefer self.env.deinit();
 
             self.executor = try self.env.newExecutionWorld();