diff --git a/src/main/java/org/codehaus/mojo/exec/ExecMojo.java b/src/main/java/org/codehaus/mojo/exec/ExecMojo.java
index 6641c6fe..9acf1b69 100644
--- a/src/main/java/org/codehaus/mojo/exec/ExecMojo.java
+++ b/src/main/java/org/codehaus/mojo/exec/ExecMojo.java
@@ -485,7 +485,7 @@ private void handleArguments( List<String> commandArguments )
                 String filePath = new File(buildDir, "modulepath").getAbsolutePath();
                 commandArguments.add( '@' + filePath );
 
-                String modulePath = StringUtils.join( computePath( (Modulepath) arguments.get( ++i ) ).iterator(), File.pathSeparator );
+                String modulePath = getModulePath(arguments.get( ++i ));
 
                 createArgFile( filePath, Arrays.asList( "-p", modulePath ) );
             }
@@ -510,6 +510,21 @@ else if ( argument instanceof Modulepath )
         }
     }
 
+    /**
+     * Builds a module path string from the given argument.
+     */
+    private String getModulePath(Object modulePathArg) throws MojoExecutionException {
+        if ( modulePathArg instanceof Modulepath ) {
+            return StringUtils.join( computePath( (Modulepath) modulePathArg ).iterator(), File.pathSeparator );
+        }
+        else if ( modulePathArg instanceof String ) {
+            return (String) modulePathArg;
+        }
+        else {
+            throw new MojoExecutionException( "Value of unsupported type for modulepath argument: "+ modulePathArg );
+        }
+    }
+
     private void fillSuccessCodes( Executor exec )
     {
         if ( successCodes != null && successCodes.length > 0 )