diff --git a/packages/schema/src/language-server/validator/expression-validator.ts b/packages/schema/src/language-server/validator/expression-validator.ts
index a4d4283e0..474f918b9 100644
--- a/packages/schema/src/language-server/validator/expression-validator.ts
+++ b/packages/schema/src/language-server/validator/expression-validator.ts
@@ -11,6 +11,7 @@ import { isDataModelFieldReference } from '@zenstackhq/sdk';
 import { ValidationAcceptor } from 'langium';
 import { isAuthInvocation, isCollectionPredicate } from '../../utils/ast-utils';
 import { AstValidator } from '../types';
+import { typeAssignable } from './utils';
 
 /**
  * Validates expressions.
@@ -124,6 +125,28 @@ export default class ExpressionValidator implements AstValidator<Expression> {
                     break;
                 }
 
+                if (
+                    (expr.left.$resolvedType?.nullable && isNullExpr(expr.right)) ||
+                    (expr.right.$resolvedType?.nullable && isNullExpr(expr.left))
+                ) {
+                    // comparing nullable field with null
+                    return;
+                }
+
+                if (
+                    typeof expr.left.$resolvedType?.decl === 'string' &&
+                    typeof expr.right.$resolvedType?.decl === 'string'
+                ) {
+                    // scalar types assignability
+                    if (
+                        !typeAssignable(expr.left.$resolvedType.decl, expr.right.$resolvedType.decl) &&
+                        !typeAssignable(expr.right.$resolvedType.decl, expr.left.$resolvedType.decl)
+                    ) {
+                        accept('error', 'incompatible operand types', { node: expr });
+                    }
+                    return;
+                }
+
                 // disallow comparing model type with scalar type or comparison between
                 // incompatible model types
                 const leftType = expr.left.$resolvedType?.decl;
diff --git a/packages/schema/src/language-server/zmodel-linker.ts b/packages/schema/src/language-server/zmodel-linker.ts
index de6afc9e5..23ada3d73 100644
--- a/packages/schema/src/language-server/zmodel-linker.ts
+++ b/packages/schema/src/language-server/zmodel-linker.ts
@@ -53,7 +53,7 @@ import {
 } from 'langium';
 import { match } from 'ts-pattern';
 import { CancellationToken } from 'vscode-jsonrpc';
-import { getAllDeclarationsFromImports } from '../utils/ast-utils';
+import { getAllDeclarationsFromImports, isAuthInvocation } from '../utils/ast-utils';
 import { mapBuiltinTypeToExpressionType } from './validator/utils';
 
 interface DefaultReference extends Reference {
@@ -337,6 +337,12 @@ export class ZModelLinker extends DefaultLinker {
             this.linkReference(node, 'member', document, [provider], true);
             if (node.member.ref) {
                 this.resolveToDeclaredType(node, node.member.ref.type);
+
+                if (node.$resolvedType && isAuthInvocation(node.operand)) {
+                    // member access on auth() function is nullable
+                    // because user may not have provided all fields
+                    node.$resolvedType.nullable = true;
+                }
             }
         }
     }
diff --git a/tests/integration/tests/regression/issue-804.test.ts b/tests/integration/tests/regression/issue-804.test.ts
new file mode 100644
index 000000000..18659b1ec
--- /dev/null
+++ b/tests/integration/tests/regression/issue-804.test.ts
@@ -0,0 +1,35 @@
+import { loadModelWithError } from '@zenstackhq/testtools';
+
+describe('Regression: issue 804', () => {
+    it('regression', async () => {
+        expect(
+            await loadModelWithError(
+                `
+        generator client {
+            provider = "prisma-client-js"
+        }
+            
+        datasource db {
+            provider = "postgresql"
+            url      = env("DATABASE_URL")
+        }
+                    
+        model User {
+            id Int @id @default(autoincrement())
+            email Int
+            posts Post[]
+        }
+          
+        model Post {
+            id Int @id @default(autoincrement())
+            author User? @relation(fields: [authorId], references: [id])
+            authorId Int
+            published Boolean
+
+            @@allow('all', auth().posts?[published] == 'TRUE')
+        }
+        `
+            )
+        ).toContain('incompatible operand types');
+    });
+});