diff --git a/packages/core/schematics/migrations/undecorated-classes-with-decorated-fields/index.ts b/packages/core/schematics/migrations/undecorated-classes-with-decorated-fields/index.ts
index 68f80e5d489d7..08738601392df 100644
--- a/packages/core/schematics/migrations/undecorated-classes-with-decorated-fields/index.ts
+++ b/packages/core/schematics/migrations/undecorated-classes-with-decorated-fields/index.ts
@@ -65,9 +65,14 @@ function runUndecoratedClassesMigration(tree: Tree, tsconfigPath: string, basePa
       file => !file.isDeclarationFile && !program.isSourceFileFromExternalLibrary(file));
 
   sourceFiles.forEach(sourceFile => {
-    const update = tree.beginUpdate(relative(basePath, sourceFile.fileName));
     const classes = getUndecoratedClassesWithDecoratedFields(sourceFile, typeChecker);
 
+    if (classes.length === 0) {
+      return;
+    }
+
+    const update = tree.beginUpdate(relative(basePath, sourceFile.fileName));
+
     classes.forEach((current, index) => {
       // If it's the first class that we're processing in this file, add `Directive` to the imports.
       if (index === 0 && !hasNamedImport(current.importDeclaration, FALLBACK_DECORATOR)) {