diff --git a/amaranth/build/plat.py b/amaranth/build/plat.py
index 293a6f1..03a6360 100644
--- a/amaranth/build/plat.py
+++ b/amaranth/build/plat.py
@@ -89,7 +89,7 @@ class Platform(ResourceManager, metaclass=ABCMeta):
         #     may fail.
         # This is OK because even if `require_tool` succeeds, the toolchain might be broken anyway.
         # The check only serves to catch common errors earlier.
-        if do_build and not self._toolchain_env_var not in os.environ:
+        if do_build and self._toolchain_env_var not in os.environ:
             for tool in self.required_tools:
                 require_tool(tool)