diff --git a/modules/openapi-generator/src/main/resources/typescript-fetch/apis.mustache b/modules/openapi-generator/src/main/resources/typescript-fetch/apis.mustache
index 6c50de8a5a6..59fa1f75d57 100644
--- a/modules/openapi-generator/src/main/resources/typescript-fetch/apis.mustache
+++ b/modules/openapi-generator/src/main/resources/typescript-fetch/apis.mustache
@@ -265,7 +265,7 @@ export class {{classname}} extends runtime.BaseAPI {
             {{/isEnumRef}}
             {{^isEnumRef}}
             {{^withoutRuntimeChecks}}
-            formParams.append('{{baseName}}', new Blob([JSON.stringify({{{dataType}}}ToJSON(requestParameters['{{paramName}}']))], { type: "application/json", }));
+            formParams.append('{{baseName}}', new Blob([JSON.stringify({{returnType}}ToJSON(requestParameters['{{paramName}}']))], { type: "application/json", }));
             {{/withoutRuntimeChecks}}{{#withoutRuntimeChecks}}
             formParams.append('{{baseName}}', new Blob([JSON.stringify(requestParameters['{{paramName}}'])], { type: "application/json", }));
             {{/withoutRuntimeChecks}}
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/typescript/fetch/TypeScriptFetchClientCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/typescript/fetch/TypeScriptFetchClientCodegenTest.java
index 0145f4890f4..d1a6d8e4301 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/typescript/fetch/TypeScriptFetchClientCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/typescript/fetch/TypeScriptFetchClientCodegenTest.java
@@ -347,6 +347,31 @@ public class TypeScriptFetchClientCodegenTest {
         TestUtils.assertFileExists(Paths.get(output + "/apis/petControllerApi.ts"));
     }
 
+    @Test(description = "Issue #20195")
+    public void givenObjectHasAdditionalPropertiesWhenGenerateThenIndexSignatureNotUsedToGenerateMethodName() throws IOException {
+        File output = Files.createTempDirectory("test").toFile().getCanonicalFile();
+        output.deleteOnExit();
+
+        TypeScriptFetchClientCodegen clientCodegen = new TypeScriptFetchClientCodegen();
+        clientCodegen.setWithoutRuntimeChecks(false);
+        clientCodegen.setOutputDir(output.getAbsolutePath());
+
+        Map<String, Object> properties = new HashMap<>();
+        properties.put(TypeScriptFetchClientCodegen.WITH_INTERFACES, true);
+        properties.put(CodegenConstants.ENUM_PROPERTY_NAMING, "original");
+        clientCodegen.additionalProperties().putAll(properties);
+
+        DefaultGenerator defaultGenerator = new DefaultGenerator();
+        defaultGenerator.opts(
+                new ClientOptInput().openAPI(TestUtils.parseSpec("src/test/resources/bugs/issue_20195.json"))
+                        .config(clientCodegen)
+        ).generate();
+
+        String outputPath = output.getAbsolutePath();
+        Path exampleApiPath = Paths.get(outputPath + "/apis/ExampleApi.ts");
+        TestUtils.assertFileContains(exampleApiPath, "new Blob([JSON.stringify(ResponseOfStringToJSON");
+    }
+
     private static File generate(Map<String, Object> properties) throws IOException {
         File output = Files.createTempDirectory("test").toFile();
         output.deleteOnExit();
diff --git a/modules/openapi-generator/src/test/resources/bugs/issue_20195.json b/modules/openapi-generator/src/test/resources/bugs/issue_20195.json
new file mode 100644
index 00000000000..46b7c00b01f
--- /dev/null
+++ b/modules/openapi-generator/src/test/resources/bugs/issue_20195.json
@@ -0,0 +1,77 @@
+{
+  "openapi": "3.0.1",
+  "info": {
+    "title": "Sample API",
+    "description": "This is sample api",
+    "version": "v1"
+  },
+  "paths": {
+    "/example_api_101": {
+      "post": {
+        "tags": [
+          "Example"
+        ],
+        "summary": "Process Auth",
+        "parameters": [
+          {
+            "name": "Id",
+            "in": "query",
+            "schema": {
+              "type": "integer",
+              "format": "int32"
+            }
+          }
+        ],
+        "requestBody": {
+          "content": {
+            "multipart/form-data": {
+              "schema": {
+                "type": "object",
+                "properties": {
+                  "paRes": {
+                    "type": "object",
+                    "additionalProperties": {
+                      "type": "string"
+                    },
+                    "description": "Form sent for Authentication"
+                  }
+                }
+              },
+              "encoding": {
+                "paRes": {
+                  "style": "form"
+                }
+              }
+            }
+          }
+        },
+        "responses": {
+          "200": {
+            "description": "OK",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ResponseOfString"
+                }
+              }
+            }
+          }
+        }
+      }
+    }
+  },
+  "components": {
+    "schemas": {
+      "ResponseOfString": {
+        "type": "object",
+        "properties": {
+          "message": {
+            "type": "string",
+            "nullable": true
+          }
+        },
+        "additionalProperties": false
+      }
+    }
+  }
+}
\ No newline at end of file
diff --git a/samples/client/petstore/typescript-fetch/tests/default/package-lock.json b/samples/client/petstore/typescript-fetch/tests/default/package-lock.json
index 609de4bb9fb..b2d105cf768 100644
--- a/samples/client/petstore/typescript-fetch/tests/default/package-lock.json
+++ b/samples/client/petstore/typescript-fetch/tests/default/package-lock.json
@@ -32,21 +32,6 @@
         "webpack": "^1.13.0"
       }
     },
-    "../../builds/with-npm-version": {
-      "name": "@openapitools/typescript-fetch-petstore",
-      "version": "1.0.0",
-      "devDependencies": {
-        "typescript": "^4.0 || ^5.0"
-      }
-    },
-    "node_modules/@openapitools/typescript-fetch-petstore": {
-      "resolved": "../../builds/with-npm-version",
-      "link": true
-    },
-    "node_modules/@swagger/typescript-fetch-petstore": {
-      "resolved": "../../builds/with-npm-version",
-      "link": true
-    },
     "node_modules/@types/chai": {
       "version": "4.3.16",
       "resolved": "https://registry.npmjs.org/@types/chai/-/chai-4.3.16.tgz",