CC to 100%.

src/plugincollection.js

@@ -283,7 +283,7 @@ export default class PluginCollection {
 
 						if ( PluginConstructor.isContextPlugin && !RequiredPluginConstructor.isContextPlugin ) {
 							throw new CKEditorError(
-								'plugincollection-context-required: Context plugin can not require plugin which is not context plugin',
+								'plugincollection-context-required: Context plugin can not require plugin which is not a context plugin',
 								null,
 								{ plugin: RequiredPluginConstructor.name, requiredBy: PluginConstructor.name }
 							);

tests/plugincollection.js

@@ -8,6 +8,7 @@
 import Editor from '../src/editor/editor';
 import PluginCollection from '../src/plugincollection';
 import Plugin from '../src/plugin';
+import ContextPlugin from '../src/contextplugin';
 import { expectToThrowCKEditorError, assertCKEditorError } from '@ckeditor/ckeditor5-utils/tests/_utils/utils';
 
 let editor, availablePlugins;
@@ -439,6 +440,25 @@ describe( 'PluginCollection', () => {
 			expect( plugins.get( PluginB ) ).to.equal( externalPlugins.get( PluginB ) ).to.instanceof( PluginB );
 			expect( plugins.get( PluginC ) ).to.instanceof( PluginC );
 		} );
+
+		it( 'should reject when context plugin requires non-context plugin', async () => {
+			class ContextPluginA extends ContextPlugin {}
+			ContextPluginA.requires = [ PluginA ];
+
+			const plugins = new PluginCollection( editor, [] );
+
+			let error;
+			const consoleErrorStub = sinon.stub( console, 'error' );
+
+			try {
+				await plugins.init( [ ContextPluginA ] );
+			} catch ( err ) {
+				error = err;
+			}
+
+			assertCKEditorError( error, /^plugincollection-context-required:/, null );
+			sinon.assert.calledOnce( consoleErrorStub );
+		} );
 	} );
 
 	describe( 'get()', () => {