diff --git a/modules/openapi-generator-maven-plugin/src/main/java/org/openapitools/codegen/plugin/CodeGenMojo.java b/modules/openapi-generator-maven-plugin/src/main/java/org/openapitools/codegen/plugin/CodeGenMojo.java
index c582cda2a913..3b5301ee51ca 100644
--- a/modules/openapi-generator-maven-plugin/src/main/java/org/openapitools/codegen/plugin/CodeGenMojo.java
+++ b/modules/openapi-generator-maven-plugin/src/main/java/org/openapitools/codegen/plugin/CodeGenMojo.java
@@ -115,14 +115,14 @@ public class CodeGenMojo extends AbstractMojo {
     /**
      * Location of the OpenAPI spec, as URL or file.
      */
-    @Parameter(name = "inputSpec", property = "openapi.generator.maven.plugin.inputSpec", required = true)
-    private String inputSpec;
+    @Parameter(name = "inputSpec", property = "openapi.generator.maven.plugin.inputSpec")
+    protected String inputSpec;
 
     /**
      * Local root folder with spec files
      */
     @Parameter(name = "inputSpecRootDirectory", property = "openapi.generator.maven.plugin.inputSpecRootDirectory")
-    private String inputSpecRootDirectory;
+    protected String inputSpecRootDirectory;
 
     /**
      * Name of the file that will contain all merged specs
@@ -557,6 +557,11 @@ public void setBuildContext(BuildContext buildContext) {
 
     @Override
     public void execute() throws MojoExecutionException {
+        if (StringUtils.isBlank(inputSpec) && StringUtils.isBlank(inputSpecRootDirectory)) {
+            LOGGER.error("inputSpec or inputSpecRootDirectory must be specified");
+            throw new MojoExecutionException("inputSpec or inputSpecRootDirectory must be specified");
+        }
+
         if (StringUtils.isNotBlank(inputSpecRootDirectory)) {
             inputSpec = new MergedSpecBuilder(inputSpecRootDirectory, mergedFileName)
                 .buildMergedSpec();
diff --git a/modules/openapi-generator-maven-plugin/src/test/java/org/openapitools/codegen/plugin/CodeGenMojoTest.java b/modules/openapi-generator-maven-plugin/src/test/java/org/openapitools/codegen/plugin/CodeGenMojoTest.java
index 9a4ebbdd8cbe..4881db21921b 100644
--- a/modules/openapi-generator-maven-plugin/src/test/java/org/openapitools/codegen/plugin/CodeGenMojoTest.java
+++ b/modules/openapi-generator-maven-plugin/src/test/java/org/openapitools/codegen/plugin/CodeGenMojoTest.java
@@ -16,6 +16,8 @@
 
 package org.openapitools.codegen.plugin;
 
+import static org.junit.Assert.assertThrows;
+
 import java.io.File;
 import java.nio.file.Files;
 import java.nio.file.Path;
@@ -31,6 +33,7 @@
 import org.apache.maven.execution.MavenExecutionRequest;
 import org.apache.maven.execution.MavenSession;
 import org.apache.maven.plugin.MojoExecution;
+import org.apache.maven.plugin.MojoExecutionException;
 import org.apache.maven.project.MavenProject;
 import org.apache.maven.project.ProjectBuilder;
 import org.apache.maven.project.ProjectBuildingRequest;
@@ -181,6 +184,36 @@ public void testCollapsedSpecAddedToArtifacts() throws Exception {
         assertEquals(1, matchingArtifacts.size());
     }
 
+    public void testAnyInputSpecMustBeProvided() throws Exception {
+        // GIVEN
+        Path folder = Files.createTempDirectory("test");
+        CodeGenMojo mojo = loadMojo(folder.toFile(), "src/test/resources/default", "executionId");
+        mojo.inputSpec = null;
+        mojo.inputSpecRootDirectory = null;
+
+        // WHEN
+        MojoExecutionException e = assertThrows(MojoExecutionException.class, mojo::execute);
+
+        // THEN
+        assertEquals("inputSpec or inputSpecRootDirectory must be specified", e.getMessage());
+    }
+
+    public void testInputSpecRootDirectoryDoesNotRequireInputSpec() throws Exception {
+        // GIVEN
+        Path folder = Files.createTempDirectory("test");
+        CodeGenMojo mojo = loadMojo(folder.toFile(), "src/test/resources/default", "executionId");
+        mojo.inputSpec = null;
+        mojo.inputSpecRootDirectory = "src/test/resources/default";
+
+        // WHEN
+        mojo.execute();
+
+        // THEN
+        /* Check the hash file was created */
+        final Path hashFolder = folder.resolve("target/generated-sources/common-maven/remote-openapi/.openapi-generator");
+        assertTrue(hashFolder.resolve("_merged_spec.yaml-executionId.sha256").toFile().exists());
+    }
+
     protected CodeGenMojo loadMojo(File temporaryFolder, String projectRoot) throws Exception {
         return loadMojo(temporaryFolder, projectRoot, "default");
     }