diff --git a/lib/compiler/passes/generate-javascript.js b/lib/compiler/passes/generate-javascript.js
index 41ba34e..6de510c 100644
--- a/lib/compiler/passes/generate-javascript.js
+++ b/lib/compiler/passes/generate-javascript.js
@@ -379,15 +379,24 @@ module.exports = function(ast, options) {
             thenLength = bc[ip + baseLength - 2],
             elseLength = bc[ip + baseLength - 1],
             baseSp     = stack.sp,
-            thenCode, elseCode;
+            thenCode, elseCode, thenSp, elseSp;
 
         ip += baseLength;
         thenCode = compile(bc.slice(ip, ip + thenLength));
+        thenSp = stack.sp;
         ip += thenLength;
+
         if (elseLength > 0) {
           stack.sp = baseSp;
           elseCode = compile(bc.slice(ip, ip + elseLength));
+          elseSp = stack.sp;
           ip += elseLength;
+
+          if (thenSp !== elseSp) {
+            throw new Error(
+              "Branches of a condition must move the stack pointer in the same way."
+            );
+          }
         }
 
         parts.push('if (' + cond + ') {');
@@ -402,12 +411,18 @@ module.exports = function(ast, options) {
       function compileLoop(cond) {
         var baseLength = 2,
             bodyLength = bc[ip + baseLength - 1],
-            bodyCode;
+            baseSp     = stack.sp,
+            bodyCode, bodySp;
 
         ip += baseLength;
         bodyCode = compile(bc.slice(ip, ip + bodyLength));
+        bodySp = stack.sp;
         ip += bodyLength;
 
+        if (bodySp !== baseSp) {
+          throw new Error("Body of a loop can't move the stack pointer.");
+        }
+
         parts.push('while (' + cond + ') {');
         parts.push(indent2(bodyCode));
         parts.push('}');