diff --git a/BUILD.gn b/BUILD.gn
index 7be30ff..6a1fa50 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1516,6 +1516,14 @@
   }
 }
 
+# TODO(cassew): Add more OS's that don't support x86.
+is_valid_x86_target =
+    target_os != "ios" && target_os != "mac" &&
+    (target_os != "linux" || use_libfuzzer || !build_with_chromium)
+assert(
+    is_valid_x86_target || target_cpu != "x86",
+    "'target_cpu=x86' is not supported for 'target_os=$target_os'. Consider omitting 'target_cpu' (default) or using 'target_cpu=x64' instead.")
+
 group("chromium_builder_perf") {
   testonly = true