diff --git a/.changeset/neat-bears-divide.md b/.changeset/neat-bears-divide.md
new file mode 100644
index 0000000000..a94b17f74d
--- /dev/null
+++ b/.changeset/neat-bears-divide.md
@@ -0,0 +1,5 @@
+---
+'@backstage/config-loader': patch
+---
+
+The `env` option of `ConfigSources.default` now correctly allows undefined members.
diff --git a/packages/config-loader/api-report.md b/packages/config-loader/api-report.md
index 835e53351d..1fef972089 100644
--- a/packages/config-loader/api-report.md
+++ b/packages/config-loader/api-report.md
@@ -88,7 +88,7 @@ export interface ConfigSourcesDefaultOptions extends BaseConfigSourcesOptions {
   // (undocumented)
   argv?: string[];
   // (undocumented)
-  env?: Record<string, string>;
+  env?: Record<string, string | undefined>;
 }
 
 // @public
diff --git a/packages/config-loader/src/sources/ConfigSources.ts b/packages/config-loader/src/sources/ConfigSources.ts
index be8892f9a9..37c1c6b087 100644
--- a/packages/config-loader/src/sources/ConfigSources.ts
+++ b/packages/config-loader/src/sources/ConfigSources.ts
@@ -107,7 +107,7 @@ export interface ConfigSourcesDefaultForTargetsOptions
  */
 export interface ConfigSourcesDefaultOptions extends BaseConfigSourcesOptions {
   argv?: string[];
-  env?: Record<string, string>;
+  env?: Record<string, string | undefined>;
 }
 
 /**