diff --git a/integration-test/diagnostics.test.ts b/integration-test/diagnostics.test.ts
index 5eb6558..f7a3d64 100644
--- a/integration-test/diagnostics.test.ts
+++ b/integration-test/diagnostics.test.ts
@@ -14,6 +14,16 @@ suite('Diagnostics', () => {
             },
         ])
     })
+
+    test('error diagnostics', async () => {
+        await testDiagnostics('error.sksl', [
+            {
+                message: "'main' must return: 'vec4', 'float4', or 'half4'",
+                range: toRange(3, 0, 3, 11),
+                severity: vscode.DiagnosticSeverity.Error,
+            },
+        ])
+    })
 })
 
 async function testDiagnostics(name: string, expected: vscode.Diagnostic[]) {
diff --git a/integration-test/fixtures/error.sksl b/integration-test/fixtures/error.sksl
new file mode 100644
index 0000000..d7d0e32
--- /dev/null
+++ b/integration-test/fixtures/error.sksl
@@ -0,0 +1,4 @@
+// kind=shader
+// 中文
+// 😋😋
+void main() {}
diff --git a/src/offset-position-converter.ts b/src/offset-position-converter.ts
index 3ae5eb7..71ff57a 100644
--- a/src/offset-position-converter.ts
+++ b/src/offset-position-converter.ts
@@ -10,7 +10,8 @@ export class OffsetPositionConverter {
 
         let index = 0
         let prev = ' '
-        for (const c of content) {
+        for (let i = 0; i < content.length; ++i) {
+            const c = content.charAt(i)
             switch (c) {
                 case '\n':
                     if (prev == '\n') {