diff --git a/packages/core/src/builder/outputBuilder.spec.ts b/packages/core/src/builder/outputBuilder.spec.ts
index d7288a6e..6e3c7d44 100644
--- a/packages/core/src/builder/outputBuilder.spec.ts
+++ b/packages/core/src/builder/outputBuilder.spec.ts
@@ -293,6 +293,18 @@ describe("Additional registers", () => {
     builder = new OutputBuilder(SAFE_MIN_BOX_VALUE, ergoTreeHex, height);
   });
 
+  it("should merge additional registers", () => {
+    builder.setAdditionalRegisters({ R4: "0101", R6: "0102" });
+    expect(builder.additionalRegisters).toEqual({ R4: "0101", R6: "0102" });
+
+    expect(() => builder.build()).toThrow(InvalidRegistersPacking); // R5 is missing
+
+    builder.setAdditionalRegisters({ R5: "0103" });
+    expect(builder.additionalRegisters).toEqual({ R4: "0101", R5: "0103", R6: "0102" });
+
+    expect(() => builder.build()).not.toThrow();
+  });
+
   it("Should bind registers properly", () => {
     builder.setAdditionalRegisters({
       R4: "0580c0fc82aa02",
@@ -335,28 +347,37 @@ describe("Additional registers", () => {
    * R9 without adding register R4 to R8, for example.
    */
   it("Should throw if some register is skipped", () => {
+    let outputBuilder = new OutputBuilder(SAFE_MIN_BOX_VALUE, ergoTreeHex, height);
     // R4 not included
     expect(() => {
-      builder.setAdditionalRegisters({
-        R5: "0580c0fc82aa02"
-      } as NonMandatoryRegisters);
+      outputBuilder
+        .setAdditionalRegisters({
+          R5: "0580c0fc82aa02"
+        } as NonMandatoryRegisters)
+        .build();
     }).toThrow(InvalidRegistersPacking);
 
+    outputBuilder = new OutputBuilder(SAFE_MIN_BOX_VALUE, ergoTreeHex, height);
     // R5 not included
     expect(() => {
-      builder.setAdditionalRegisters({
-        R4: "0580c0fc82aa02",
-        R6: "0580c0fc82aa02"
-      } as NonMandatoryRegisters);
+      outputBuilder
+        .setAdditionalRegisters({
+          R4: "0580c0fc82aa02",
+          R6: "0580c0fc82aa02"
+        } as NonMandatoryRegisters)
+        .build();
     }).toThrow(InvalidRegistersPacking);
 
+    outputBuilder = new OutputBuilder(SAFE_MIN_BOX_VALUE, ergoTreeHex, height);
     // R5 explicitly set to undefined
     expect(() => {
-      builder.setAdditionalRegisters({
-        R4: "0580c0fc82aa02",
-        R5: undefined,
-        R6: "0580c0fc82aa02"
-      } as NonMandatoryRegisters);
+      outputBuilder
+        .setAdditionalRegisters({
+          R4: "0580c0fc82aa02",
+          R5: undefined,
+          R6: "0580c0fc82aa02"
+        } as NonMandatoryRegisters)
+        .build();
     }).toThrow(InvalidRegistersPacking);
   });
 });
diff --git a/packages/core/src/builder/outputBuilder.ts b/packages/core/src/builder/outputBuilder.ts
index 9cd34f83..79638a01 100644
--- a/packages/core/src/builder/outputBuilder.ts
+++ b/packages/core/src/builder/outputBuilder.ts
@@ -167,17 +167,13 @@ export class OutputBuilder {
   }
 
   setAdditionalRegisters(registers: NonMandatoryRegisters<ConstantInput>): OutputBuilder {
-    const hexRegisters: NonMandatoryRegisters = {};
     for (const key in registers) {
       const r = registers[key as keyof NonMandatoryRegisters];
       if (!r) continue;
 
-      hexRegisters[key as keyof NonMandatoryRegisters] = typeof r === "string" ? r : r.toHex();
+      this.#registers[key as keyof NonMandatoryRegisters] = typeof r === "string" ? r : r.toHex();
     }
 
-    if (!areRegistersDenselyPacked(hexRegisters)) throw new InvalidRegistersPacking();
-    this.#registers = hexRegisters;
-
     return this;
   }
 
@@ -187,8 +183,9 @@ export class OutputBuilder {
   }
 
   build(transactionInputs?: UnsignedInput[] | Box<Amount>[]): ErgoBoxCandidate {
-    let tokens: TokenAmount<bigint>[];
+    if (!areRegistersDenselyPacked(this.#registers)) throw new InvalidRegistersPacking();
 
+    let tokens: TokenAmount<bigint>[];
     if (this.minting) {
       const mintingTokenId = transactionInputs ? transactionInputs[0]?.boxId : undefined;
       tokens = this.assets.toArray(mintingTokenId);