diff --git a/lib/options.json b/lib/options.json
index 554f8fe48d..d546d03f37 100644
--- a/lib/options.json
+++ b/lib/options.json
@@ -523,7 +523,8 @@
           "type": "number"
         },
         {
-          "type": "string"
+          "type": "string",
+          "minLength": 1
         },
         {
           "enum": ["auto"]
diff --git a/test/__snapshots__/validate-options.test.js.snap.webpack4 b/test/__snapshots__/validate-options.test.js.snap.webpack4
index a69d352d81..a4f107f387 100644
--- a/test/__snapshots__/validate-options.test.js.snap.webpack4
+++ b/test/__snapshots__/validate-options.test.js.snap.webpack4
@@ -414,25 +414,30 @@ exports[`options validate should throw an error on the "open" option with '{"tar
        * configuration.open.target should be a non-empty string."
 `;
 
+exports[`options validate should throw an error on the "port" option with '' value 1`] = `
+"ValidationError: Invalid configuration object. Object has been initialized using a configuration object that does not match the API schema.
+ - configuration.port should be an non-empty string."
+`;
+
 exports[`options validate should throw an error on the "port" option with 'false' value 1`] = `
 "ValidationError: Invalid configuration object. Object has been initialized using a configuration object that does not match the API schema.
  - configuration.port should be one of these:
-   number | string | \\"auto\\"
+   number | non-empty string | \\"auto\\"
    -> Specify a port number to listen for requests on. https://webpack.js.org/configuration/dev-server/#devserverport
    Details:
     * configuration.port should be a number.
-    * configuration.port should be a string.
+    * configuration.port should be a non-empty string.
     * configuration.port should be \\"auto\\"."
 `;
 
 exports[`options validate should throw an error on the "port" option with 'null' value 1`] = `
 "ValidationError: Invalid configuration object. Object has been initialized using a configuration object that does not match the API schema.
  - configuration.port should be one of these:
-   number | string | \\"auto\\"
+   number | non-empty string | \\"auto\\"
    -> Specify a port number to listen for requests on. https://webpack.js.org/configuration/dev-server/#devserverport
    Details:
     * configuration.port should be a number.
-    * configuration.port should be a string.
+    * configuration.port should be a non-empty string.
     * configuration.port should be \\"auto\\"."
 `;
 
diff --git a/test/__snapshots__/validate-options.test.js.snap.webpack5 b/test/__snapshots__/validate-options.test.js.snap.webpack5
index a69d352d81..a4f107f387 100644
--- a/test/__snapshots__/validate-options.test.js.snap.webpack5
+++ b/test/__snapshots__/validate-options.test.js.snap.webpack5
@@ -414,25 +414,30 @@ exports[`options validate should throw an error on the "open" option with '{"tar
        * configuration.open.target should be a non-empty string."
 `;
 
+exports[`options validate should throw an error on the "port" option with '' value 1`] = `
+"ValidationError: Invalid configuration object. Object has been initialized using a configuration object that does not match the API schema.
+ - configuration.port should be an non-empty string."
+`;
+
 exports[`options validate should throw an error on the "port" option with 'false' value 1`] = `
 "ValidationError: Invalid configuration object. Object has been initialized using a configuration object that does not match the API schema.
  - configuration.port should be one of these:
-   number | string | \\"auto\\"
+   number | non-empty string | \\"auto\\"
    -> Specify a port number to listen for requests on. https://webpack.js.org/configuration/dev-server/#devserverport
    Details:
     * configuration.port should be a number.
-    * configuration.port should be a string.
+    * configuration.port should be a non-empty string.
     * configuration.port should be \\"auto\\"."
 `;
 
 exports[`options validate should throw an error on the "port" option with 'null' value 1`] = `
 "ValidationError: Invalid configuration object. Object has been initialized using a configuration object that does not match the API schema.
  - configuration.port should be one of these:
-   number | string | \\"auto\\"
+   number | non-empty string | \\"auto\\"
    -> Specify a port number to listen for requests on. https://webpack.js.org/configuration/dev-server/#devserverport
    Details:
     * configuration.port should be a number.
-    * configuration.port should be a string.
+    * configuration.port should be a non-empty string.
     * configuration.port should be \\"auto\\"."
 `;
 
diff --git a/test/validate-options.test.js b/test/validate-options.test.js
index c5db1a006e..c6e37b72c2 100644
--- a/test/validate-options.test.js
+++ b/test/validate-options.test.js
@@ -258,8 +258,8 @@ const tests = {
     failure: ['', [], { foo: 'bar' }, { target: 90 }, { app: true }],
   },
   port: {
-    success: ['', 0, 'auto'],
-    failure: [false, null],
+    success: ['8080', 8080, 'auto'],
+    failure: [false, null, ''],
   },
   proxy: {
     success: [