diff --git a/tools/push3-transpiler/src/transpiler.ts b/tools/push3-transpiler/src/transpiler.ts
index 198561a..e1625cd 100644
--- a/tools/push3-transpiler/src/transpiler.ts
+++ b/tools/push3-transpiler/src/transpiler.ts
@@ -255,8 +255,24 @@ function processExecIf(
   processItems(toItems(falseBranch), falseState);
   state.varCounter = falseState.varCounter;
 
+  // --- Validate branch stack depths match ---
+  if (trueState.dStack.length !== falseState.dStack.length) {
+    throw new Error(
+      `EXEC.IF: DYADIC stack depth mismatch — ` +
+      `true branch produces ${trueState.dStack.length} values, ` +
+      `false branch produces ${falseState.dStack.length}`
+    );
+  }
+  if (trueState.bStack.length !== falseState.bStack.length) {
+    throw new Error(
+      `EXEC.IF: BOOLEAN stack depth mismatch — ` +
+      `true branch produces ${trueState.bStack.length} values, ` +
+      `false branch produces ${falseState.bStack.length}`
+    );
+  }
+
   // --- Compare dStacks elementwise ---
-  const maxDLen = Math.max(trueState.dStack.length, falseState.dStack.length);
+  const maxDLen = trueState.dStack.length; // both are equal after validation
   // result var for each position that changed or is new
   const dResultMap = new Map<number, string>(); // position → result var name
   for (let k = 0; k < maxDLen; k++) {
@@ -272,7 +288,7 @@ function processExecIf(
   }
 
   // --- Compare bStacks elementwise ---
-  const maxBLen = Math.max(trueState.bStack.length, falseState.bStack.length);
+  const maxBLen = trueState.bStack.length; // both are equal after validation
   const bResultMap = new Map<number, string>();
   for (let k = 0; k < maxBLen; k++) {
     const tv = trueState.bStack[k];
@@ -296,11 +312,11 @@ function processExecIf(
     const pad = '    '.repeat(indentLevel);
     const assignments: string[] = [];
     for (const [k, rv] of dMap) {
-      const val = branchState.dStack[k] ?? '0';
+      const val = branchState.dStack[k]!;
       assignments.push(`${pad}${rv} = uint256(${val});`);
     }
     for (const [k, rv] of bMap) {
-      const val = branchState.bStack[k] ?? 'false';
+      const val = branchState.bStack[k]!;
       assignments.push(`${pad}${rv} = ${val};`);
     }
     return assignments;
@@ -326,12 +342,12 @@ function processExecIf(
   const newDStack: string[] = [];
   for (let k = 0; k < maxDLen; k++) {
     const rv = dResultMap.get(k);
-    newDStack.push(rv ?? (dBefore[k] ?? trueState.dStack[k] ?? '0'));
+    newDStack.push(rv ?? (dBefore[k] ?? trueState.dStack[k]!));
   }
   const newBStack: string[] = [];
   for (let k = 0; k < maxBLen; k++) {
     const rv = bResultMap.get(k);
-    newBStack.push(rv ?? (bBefore[k] ?? trueState.bStack[k] ?? 'false'));
+    newBStack.push(rv ?? (bBefore[k] ?? trueState.bStack[k]!));
   }
 
   state.dStack = newDStack;
diff --git a/tools/push3-transpiler/test/transpiler.test.ts b/tools/push3-transpiler/test/transpiler.test.ts
index 4a53f44..045c272 100644
--- a/tools/push3-transpiler/test/transpiler.test.ts
+++ b/tools/push3-transpiler/test/transpiler.test.ts
@@ -188,8 +188,7 @@ describe('EXEC.IF balanced branches', () => {
 // ---------------------------------------------------------------------------
 
 describe('EXEC.IF unbalanced branches', () => {
-  it('true branch pushes more values than false branch', () => {
-    // True branch pushes 2 values, false branch pushes 1
+  it('true branch pushes more DYADIC values than false branch → error', () => {
     const src = `(
       DYADIC.POP DYADIC.POP DYADIC.POP DYADIC.POP
       DYADIC.POP DYADIC.POP DYADIC.POP DYADIC.POP
@@ -198,13 +197,10 @@ describe('EXEC.IF unbalanced branches', () => {
       (10 20)
       (10)
     )`;
-    const result = run(src);
-    const body = result.functionBody.join('\n');
-    assert.ok(body.includes('if ('), 'should emit if/else');
-    // The missing value in false branch defaults to '0'
+    assert.throws(() => run(src), /DYADIC stack depth mismatch/);
   });
 
-  it('false branch pushes more values than true branch', () => {
+  it('false branch pushes more DYADIC values than true branch → error', () => {
     const src = `(
       DYADIC.POP DYADIC.POP DYADIC.POP DYADIC.POP
       DYADIC.POP DYADIC.POP DYADIC.POP DYADIC.POP
@@ -213,24 +209,33 @@ describe('EXEC.IF unbalanced branches', () => {
       (10)
       (10 20)
     )`;
-    const result = run(src);
-    const body = result.functionBody.join('\n');
-    assert.ok(body.includes('if ('), 'should emit if/else');
+    assert.throws(() => run(src), /DYADIC stack depth mismatch/);
   });
 
-  it('one branch is empty (no-op)', () => {
+  it('error message includes both branch depths', () => {
+    const src = `(
+      DYADIC.POP DYADIC.POP DYADIC.POP DYADIC.POP
+      DYADIC.POP DYADIC.POP DYADIC.POP DYADIC.POP
+      100 50 DYADIC.>
+      EXEC.IF
+      (10 20 30)
+      (10)
+    )`;
+    assert.throws(() => run(src), /true branch produces 3.*false branch produces 1/);
+  });
+
+  it('one branch pushes extra, the other is no-op → depth mismatch error', () => {
+    // True branch pushes 2 extra values, false branch leaves stack unchanged
     const src = `(
       DYADIC.POP DYADIC.POP DYADIC.POP DYADIC.POP
       DYADIC.POP DYADIC.POP DYADIC.POP DYADIC.POP
       50
       100 50 DYADIC.>
       EXEC.IF
-      (DYADIC.POP 99)
+      (99 88)
       ()
     )`;
-    const result = run(src);
-    const body = result.functionBody.join('\n');
-    assert.ok(body.includes('if ('), 'should emit if/else');
+    assert.throws(() => run(src), /DYADIC stack depth mismatch/);
   });
 
   it('EXEC.IF with missing branches throws', () => {
@@ -240,6 +245,20 @@ describe('EXEC.IF unbalanced branches', () => {
     )`;
     assert.throws(() => run(src), /EXEC\.IF: missing branches/);
   });
+
+  it('BOOLEAN stack depth mismatch between branches → error', () => {
+    // true branch pushes a boolean comparison, false branch does not
+    const src = `(
+      DYADIC.POP DYADIC.POP DYADIC.POP DYADIC.POP
+      DYADIC.POP DYADIC.POP DYADIC.POP DYADIC.POP
+      10 20 30 40
+      TRUE
+      EXEC.IF
+      (10 5 DYADIC.>)
+      ()
+    )`;
+    assert.throws(() => run(src), /BOOLEAN stack depth mismatch/);
+  });
 });
 
 // ---------------------------------------------------------------------------