diff --git a/lib/binary_parser.ts b/lib/binary_parser.ts
index 111fd3b..caf7a1b 100644
--- a/lib/binary_parser.ts
+++ b/lib/binary_parser.ts
@@ -1,7 +1,7 @@
 class Context {
   code = "";
   scopes = [["vars"]];
-  bitFields: Parser[] = [];
+  bitFields: (Parser & { thisVar: string })[] = [];
   tmpVariableCount = 0;
   references = new Map<string, { resolved: boolean; requested: boolean }>();
   importPath: string;
@@ -1018,13 +1018,14 @@ export class Parser {
     return this.generateNext(ctx);
   }
 
-  private generateAssert(ctx: Context) {
+  private generateAssert(
+    ctx: Context,
+    varName = ctx.generateVariable(this.varName),
+  ) {
     if (!this.options.assert) {
       return;
     }
 
-    const varName = ctx.generateVariable(this.varName);
-
     switch (typeof this.options.assert) {
       case "function":
         {
@@ -1093,6 +1094,7 @@ export class Parser {
     parser.generateAssert = this.generateAssert.bind(this);
     parser.generateFormatter = this.generateFormatter.bind(this);
     parser.varName = ctx.generateVariable(parser.varName);
+    parser.thisVar = ctx.generateVariable();
     ctx.bitFields.push(parser);
 
     if (!this.next || this.nextNotBit()) {
@@ -1166,7 +1168,7 @@ export class Parser {
         }
 
         if (parser.options.assert) {
-          parser.generateAssert(ctx);
+          parser.generateAssert(ctx, parser.varName);
         }
 
         if (parser.options.formatter) {
@@ -1174,6 +1176,7 @@ export class Parser {
             ctx,
             parser.varName,
             parser.options.formatter,
+            parser.thisVar,
           );
         }
 
@@ -1557,12 +1560,11 @@ export class Parser {
     ctx: Context,
     varName: string,
     formatter: Function,
+    thisVar = ctx.generateVariable(),
   ) {
     if (typeof formatter === "function") {
       const func = ctx.addImport(formatter);
-      ctx.pushCode(
-        `${varName} = ${func}.call(${ctx.generateVariable()}, ${varName});`,
-      );
+      ctx.pushCode(`${varName} = ${func}.call(${thisVar}, ${varName});`);
     }
   }
 
diff --git a/test/primitive_parser.ts b/test/primitive_parser.ts
index 66f2489..25659ec 100644
--- a/test/primitive_parser.ts
+++ b/test/primitive_parser.ts
@@ -303,6 +303,22 @@ function primitiveParserTests(
         expect(parser.parse(buf)).toStrictEqual({ a: 0xa, b: 0xbc, c: 0xd });
         expect(options).toStrictEqual({});
       });
+
+      it("should assert and format bit fields spanning into a nest", () => {
+        let self: any;
+        const parser = new Parser()
+          .bit4("a", {
+            assert: 0xa,
+            formatter: function (v) {
+              self = this;
+              return v;
+            },
+          })
+          .nest("n", { type: new Parser().bit4("b") });
+        const result = parser.parse(binaryLiteral("1010 1011"));
+        expect(result).toStrictEqual({ a: 0xa, n: { b: 0xb } });
+        expect(self).toBe(result);
+      });
     });
 
     describe("String parser", () => {