diff --git a/tsconfig.build.json b/tsconfig.build.json
index 6518ac9..20b354f 100644
--- a/tsconfig.build.json
+++ b/tsconfig.build.json
@@ -9,6 +9,9 @@
     "module": "ESNext",
     "outDir": "dist",
     "rootDir": "src",
+    // Restated (not just inherited) so the published build fails loudly if the
+    // base config ever loses it.
+    "erasableSyntaxOnly": true,
     "declaration": true,
     "declarationMap": true,
     "sourceMap": true,
diff --git a/tsconfig.json b/tsconfig.json
index 62c82d0..57f1a51 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -28,6 +28,10 @@
     "noPropertyAccessFromIndexSignature": true,
     "exactOptionalPropertyTypes": true,
     "noImplicitReturns": true,
+    // Reject the non-erasable TypeScript constructs (enums, namespaces,
+    // constructor parameter properties, `import =`). This library ships its
+    // source, and downstream consumers type-strip rather than compile it.
+    "erasableSyntaxOnly": true,
 
     // Cache
     "incremental": true,