diff --git a/extensions/vscode-api-tests/src/commands.test.ts b/extensions/vscode-api-tests/src/commands.test.ts
index 182bc2f26a2..043e375cd6a 100644
--- a/extensions/vscode-api-tests/src/commands.test.ts
+++ b/extensions/vscode-api-tests/src/commands.test.ts
@@ -72,7 +72,7 @@ suite('commands namespace tests', () => {
 		let virtualDocumentUri = Uri.parse('speciale://authority/path');
 		let title = 'A title';
 
-		return commands.executeCommand('vscode.previewHtml', virtualDocumentUri, title).then(success => {
+		return commands.executeCommand('vscode.previewHtml', virtualDocumentUri, ViewColumn.Three, title).then(success => {
 			assert.ok(success);
 			registration.dispose();
 		});
diff --git a/src/vs/workbench/api/node/extHostApiCommands.ts b/src/vs/workbench/api/node/extHostApiCommands.ts
index bb823b1ef82..e08410ddfbf 100644
--- a/src/vs/workbench/api/node/extHostApiCommands.ts
+++ b/src/vs/workbench/api/node/extHostApiCommands.ts
@@ -170,7 +170,7 @@ class ExtHostApiCommands {
 				`,
 			args: [
 				{ name: 'uri', description: 'Uri of the resource to preview.', constraint: value => value instanceof URI || typeof value === 'string' },
-				{ name: 'column', description: '(optional) Column in which to preview.' },
+				{ name: 'column', description: '(optional) Column in which to preview.', constraint: value => typeof value === 'undefined' || (typeof value === 'number' && typeof types.ViewColumn[value] === 'string') },
 				{ name: 'label', description: '(optional) An human readable string that is used as title for the preview.', constraint: v => typeof v === 'string' || typeof v === 'undefined' }
 			]
 		});
diff --git a/src/vs/workbench/api/node/extHostCommands.ts b/src/vs/workbench/api/node/extHostCommands.ts
index 8443632ce02..5d85a32f694 100644
--- a/src/vs/workbench/api/node/extHostCommands.ts
+++ b/src/vs/workbench/api/node/extHostCommands.ts
@@ -85,13 +85,20 @@ export class ExtHostCommands {
 		if (!command) {
 			return Promise.reject<T>(`Contributed command '${id}' does not exist.`);
 		}
-		try {
-			let {callback, thisArg, description} = command;
-			if (description) {
-				for (let i = 0; i < description.args.length; i++) {
+
+		let {callback, thisArg, description} = command;
+
+		if (description) {
+			for (let i = 0; i < description.args.length; i++) {
+				try {
 					validateConstraint(args[i], description.args[i].constraint);
+				} catch (err) {
+					return Promise.reject<T>(`Running the contributed command:'${id}' failed. Illegal argument '${description.args[i].name}' - ${description.args[i].description}`);
 				}
 			}
+		}
+
+		try {
 			let result = callback.apply(thisArg, args);
 			return Promise.resolve(result);
 		} catch (err) {