diff --git a/mod.ts b/mod.ts
index c80648a..07933aa 100644
--- a/mod.ts
+++ b/mod.ts
@@ -30,9 +30,9 @@ import { getLogger } from "@std/log";
 export class EnvNotSetError extends Error {
   /**
    * Create a new {@link EnvNotSetError}.
-   * 
+   *
    * Use this when an environment variable is not set and the application tries to access it.
-   * 
+   *
    * @param envVariable name of the environment variable
    * @param cause the cause of the error, if any
    */
@@ -244,7 +244,7 @@ function logger() {
  * A type that is compatible with Zod schemas.
  */
 export type ZodSchemaCompat = {
-  safeParse: (value?: string) => { error: Error | null | undefined };
+  safeParse: (value?: string) => { error?: Error };
   isOptional: () => boolean;
 };
 
@@ -271,9 +271,9 @@ export const REQUIRED: ZodSchemaCompat = {
 
 /**
  * A `ZodSchemaCompat` validator that represents an optional variable.
- * 
+ *
  * Useful for projects where you don't need full-blown Zod schemas.
- * 
+ *
  * @example
  * ```typescript
  * import { initVariable, OPTIONAL } from "@wuespace/envar/";
@@ -292,9 +292,9 @@ export const OPTIONAL: ZodSchemaCompat = {
 
 /**
  * A `ZodSchemaCompat` validator that represents a required, non-empty variable.
- * 
+ *
  * Useful for projects where you don't need full-blown Zod schemas.
- * 
+ *
  * @example
  * ```typescript
  * import { initVariable, REQUIRED_NON_EMPTY } from "@wuespace/envar/";
@@ -307,16 +307,18 @@ export const REQUIRED_NON_EMPTY: ZodSchemaCompat = {
   safeParse: (val) => ({
     error: typeof val === "string" && val.length > 0
       ? undefined
-      : new Error(`Expected value to be a non-empty string, but got "${val?.toString()}"`),
+      : new Error(
+        `Expected value to be a non-empty string, but got "${val?.toString()}"`,
+      ),
   }),
 };
 
 /**
  * A `ZodSchemaCompat` validator that represents an optional, non-empty variable.
  * Valid values are non-empty strings or undefined. Any other value is invalid.
- * 
+ *
  * Useful for projects where you don't need full-blown Zod schemas.
- * 
+ *
  * @example
  * ```typescript
  * import { initVariable, OPTIONAL_NON_EMPTY } from "@wuespace/envar/";
@@ -328,6 +330,8 @@ export const OPTIONAL_NON_EMPTY: ZodSchemaCompat = {
   safeParse: (val) => ({
     error: typeof val === "string" && val.length > 0 || val === undefined
       ? undefined
-      : new Error(`Expected value to be a non-empty string or unset, but got "${val?.toString()}"`),
+      : new Error(
+        `Expected value to be a non-empty string or unset, but got "${val?.toString()}"`,
+      ),
   }),
 };
diff --git a/mod_test.ts b/mod_test.ts
index 4c5ba6f..d4e831d 100644
--- a/mod_test.ts
+++ b/mod_test.ts
@@ -46,7 +46,7 @@ const PARSE_ERROR = new Error("parse error");
 
 const OPTIONAL_PASSING = {
 	isOptional: () => true,
-	safeParse: () => ({ error: null }),
+	safeParse: () => ({ error: undefined }),
 };
 const OPTIONAL_FAILING = {
 	isOptional: () => true,
@@ -55,7 +55,7 @@ const OPTIONAL_FAILING = {
 const REQUIRED_PASSING = {
 	isOptional: () => false,
 	safeParse: (val: unknown) => ({
-		error: typeof val === "string" ? null : PARSE_ERROR,
+		error: typeof val === "string" ? undefined : PARSE_ERROR,
 	}),
 };
 const REQUIRED_FAILING = {