diff --git a/packages/babel-core/src/config/caching.js b/packages/babel-core/src/config/caching.js
index 7c3a28bda2..c8db5a166c 100644
--- a/packages/babel-core/src/config/caching.js
+++ b/packages/babel-core/src/config/caching.js
@@ -206,12 +206,29 @@ function makeSimpleConfigurator(
       return;
     }
 
-    return cache.using(val);
+    return cache.using(() => assertSimpleType(val()));
   }
   cacheFn.forever = () => cache.forever();
   cacheFn.never = () => cache.never();
-  cacheFn.using = cb => cache.using(() => cb());
-  cacheFn.invalidate = cb => cache.invalidate(() => cb());
+  cacheFn.using = cb => cache.using(() => assertSimpleType(cb()));
+  cacheFn.invalidate = cb => cache.invalidate(() => assertSimpleType(cb()));
 
   return (cacheFn: any);
 }
+
+// Types are limited here so that in the future these values can be used
+// as part of Babel's caching logic.
+type SimpleType = string | boolean | number | null | void;
+export function assertSimpleType(value: mixed): SimpleType {
+  if (
+    value != null &&
+    typeof value !== "string" &&
+    typeof value !== "boolean" &&
+    typeof value !== "number"
+  ) {
+    throw new Error(
+      "Cache keys must be either string, boolean, number, null, or undefined.",
+    );
+  }
+  return value;
+}
diff --git a/packages/babel-core/src/config/helpers/config-api.js b/packages/babel-core/src/config/helpers/config-api.js
index da14c22eeb..a50320680f 100644
--- a/packages/babel-core/src/config/helpers/config-api.js
+++ b/packages/babel-core/src/config/helpers/config-api.js
@@ -2,7 +2,11 @@
 
 import semver from "semver";
 import { version as coreVersion } from "../../";
-import type { CacheConfigurator, SimpleCacheConfigurator } from "../caching";
+import {
+  assertSimpleType,
+  type CacheConfigurator,
+  type SimpleCacheConfigurator,
+} from "../caching";
 
 type EnvFunction = {
   (): string,
@@ -25,7 +29,9 @@ export default function makeAPI(
   const env: any = value =>
     cache.using(data => {
       if (typeof value === "undefined") return data.envName;
-      if (typeof value === "function") return value(data.envName);
+      if (typeof value === "function") {
+        return assertSimpleType(value(data.envName));
+      }
       if (!Array.isArray(value)) value = [value];
 
       return value.some(entry => {