diff --git a/packages/config-loader/src/sources/MergedConfigSource.ts b/packages/config-loader/src/sources/MergedConfigSource.ts
index 206290b383..2b67bb7080 100644
--- a/packages/config-loader/src/sources/MergedConfigSource.ts
+++ b/packages/config-loader/src/sources/MergedConfigSource.ts
@@ -29,7 +29,7 @@ export class MergedConfigSource implements ConfigSource {
   // An optimization to flatten nested merged sources to avid unnecessary microtasks
   static #flattenSources(sources: ConfigSource[]): ConfigSource[] {
     return sources.flatMap(source => {
-      if (sourcesSymbol in source) {
+      if (sourcesSymbol in source && Array.isArray(source[sourcesSymbol])) {
         return this.#flattenSources(source[sourcesSymbol] as ConfigSource[]);
       }
       return source;