diff --git a/modules/openapi-generator/src/main/resources/scala-sttp/paramCreation.mustache b/modules/openapi-generator/src/main/resources/scala-sttp/paramCreation.mustache
index 25ec73e8d5e1..a0c90180947b 100644
--- a/modules/openapi-generator/src/main/resources/scala-sttp/paramCreation.mustache
+++ b/modules/openapi-generator/src/main/resources/scala-sttp/paramCreation.mustache
@@ -1 +1 @@
-"{{baseName}}", {{#isContainer}}ArrayValues({{{paramName}}}{{#collectionFormat}}, {{collectionFormat.toUpperCase}}{{/collectionFormat}}){{/isContainer}}{{^isContainer}}{{{paramName}}}{{/isContainer}}.toString
\ No newline at end of file
+"{{baseName}}", {{#isContainer}}ArrayValues({{{paramName}}}{{#collectionFormat}}, {{collectionFormat.toUpperCase}}{{/collectionFormat}}){{/isContainer}}{{^isContainer}}{{{paramName}}}{{/isContainer}}{{#required}}.toString{{/required}}{{^required}}.map(_.toString()){{/required}}
\ No newline at end of file
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/scala/SttpCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/scala/SttpCodegenTest.java
index 7b745461cf85..c6f2150321bd 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/scala/SttpCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/scala/SttpCodegenTest.java
@@ -110,4 +110,42 @@ public void verifyApiKeyLocations() throws IOException {
         assertFileContains(path, ".cookie(\"apikey\", apiKeyCookie)");
     }
 
+    @Test
+    public void headerSerialization() throws IOException {
+        File output = Files.createTempDirectory("test").toFile().getCanonicalFile();
+        output.deleteOnExit();
+        String outputPath = output.getAbsolutePath().replace('\\', '/');
+
+        OpenAPI openAPI = new OpenAPIParser()
+                .readLocation("src/test/resources/bugs/issue_21602.yaml", null, new ParseOptions()).getOpenAPI();
+
+        ScalaSttpClientCodegen codegen = new ScalaSttpClientCodegen();
+        codegen.setOutputDir(output.getAbsolutePath());
+        codegen.additionalProperties().put(CXFServerFeatures.LOAD_TEST_DATA_FROM_FILE, "true");
+
+        ClientOptInput input = new ClientOptInput();
+        input.openAPI(openAPI);
+        input.config(codegen);
+
+        DefaultGenerator generator = new DefaultGenerator();
+
+        generator.setGeneratorPropertyDefault(CodegenConstants.MODELS, "true");
+        generator.setGeneratorPropertyDefault(CodegenConstants.MODEL_TESTS, "false");
+        generator.setGeneratorPropertyDefault(CodegenConstants.MODEL_DOCS, "false");
+        generator.setGeneratorPropertyDefault(CodegenConstants.APIS, "true");
+        generator.setGeneratorPropertyDefault(CodegenConstants.SUPPORTING_FILES, "false");
+        generator.opts(input).generate();
+
+        Path path = Paths.get(outputPath + "/src/main/scala/org/openapitools/client/api/DefaultApi.scala");
+        assertFileContains(path, ".method(Method.GET, uri\"$baseUrl/ping\")\n");
+        assertFileContains(path, "xOptionalHeader: Option[String] = None");
+        assertFileContains(path, ".header(\"X-Optional-Header\", xOptionalHeader.map(_.toString()))");
+        assertFileContains(path, "xRequiredHeader: String");
+        assertFileContains(path, ".header(\"X-Required-Header\", xRequiredHeader.toString)");
+        assertFileContains(path, "xOptionalSchemaHeader: Option[UUID] = None");
+        assertFileContains(path, ".header(\"X-Optional-Schema-Header\", xOptionalSchemaHeader.map(_.toString()))");
+        assertFileContains(path, "xRequiredSchemaHeader: UUID");
+        assertFileContains(path, ".header(\"X-Required-Schema-Header\", xRequiredSchemaHeader.toString)");
+    }
+
 }
diff --git a/modules/openapi-generator/src/test/resources/bugs/issue_21602.yaml b/modules/openapi-generator/src/test/resources/bugs/issue_21602.yaml
new file mode 100644
index 000000000000..928b5603abe2
--- /dev/null
+++ b/modules/openapi-generator/src/test/resources/bugs/issue_21602.yaml
@@ -0,0 +1,44 @@
+openapi: "3.0.0"
+info:
+  title: Optional Header Test
+  version: 1.0.0
+paths:
+  /ping:
+    get:
+      summary: Ping with optional header
+      operationId: getPing
+      parameters:
+        - name: X-Optional-Header
+          in: header
+          required: false
+          schema:
+            type: string
+        - name: X-Required-Header
+          in: header
+          required: true
+          schema:
+            type: string
+        - name: X-Optional-Schema-Header
+          in: header
+          required: false
+          schema:
+            $ref: '#/components/schemas/UUID'
+        - name: X-Required-Schema-Header
+          in: header
+          required: true
+          schema:
+            $ref: '#/components/schemas/UUID'
+      responses:
+        '200':
+          description: Success
+          content:
+            application/json:
+              schema:
+                type: string
+components:
+  schemas:
+    UUID:
+      type: object
+      properties:
+        uuid:
+          type: string
\ No newline at end of file
diff --git a/samples/client/petstore/scala-sttp-circe/src/main/scala/org/openapitools/client/api/PetApi.scala b/samples/client/petstore/scala-sttp-circe/src/main/scala/org/openapitools/client/api/PetApi.scala
index dc45f1f8c72d..663261959b93 100644
--- a/samples/client/petstore/scala-sttp-circe/src/main/scala/org/openapitools/client/api/PetApi.scala
+++ b/samples/client/petstore/scala-sttp-circe/src/main/scala/org/openapitools/client/api/PetApi.scala
@@ -55,7 +55,7 @@ class PetApi(baseUrl: String) {
     basicRequest
       .method(Method.DELETE, uri"$baseUrl/pet/${petId}")
       .contentType("application/json")
-      .header("api_key", apiKey.toString)
+      .header("api_key", apiKey.map(_.toString()))
       .response(asString.mapWithMetadata(ResponseAs.deserializeRightWithError(_ => Right(()))))
 
   /**
diff --git a/samples/client/petstore/scala-sttp/src/main/scala/org/openapitools/client/api/PetApi.scala b/samples/client/petstore/scala-sttp/src/main/scala/org/openapitools/client/api/PetApi.scala
index dc45f1f8c72d..663261959b93 100644
--- a/samples/client/petstore/scala-sttp/src/main/scala/org/openapitools/client/api/PetApi.scala
+++ b/samples/client/petstore/scala-sttp/src/main/scala/org/openapitools/client/api/PetApi.scala
@@ -55,7 +55,7 @@ class PetApi(baseUrl: String) {
     basicRequest
       .method(Method.DELETE, uri"$baseUrl/pet/${petId}")
       .contentType("application/json")
-      .header("api_key", apiKey.toString)
+      .header("api_key", apiKey.map(_.toString()))
       .response(asString.mapWithMetadata(ResponseAs.deserializeRightWithError(_ => Right(()))))
 
   /**