--- a/js/src/ion/IonBuilder.cpp
+++ b/js/src/ion/IonBuilder.cpp
@@ -1563,17 +1563,17 @@ IonBuilder::jsop_localinc(JSOp op)
     current->pushLocal(GET_SLOTNO(pc));
 
     if (!pushConstant(Int32Value(amt)))
         return false;
 
     if (!jsop_binary(JSOP_ADD))
         return false;
 
-    if (current->setLocal(GET_SLOTNO(pc)))
+    if (!current->setLocal(GET_SLOTNO(pc)))
         return false;
 
     if (post_incr)
         current->pop();
 
     return true;
 }
 
@@ -1589,17 +1589,17 @@ IonBuilder::jsop_arginc(JSOp op)
     current->pushArg(GET_SLOTNO(pc));
 
     if (!pushConstant(Int32Value(amt)))
         return false;
 
     if (!jsop_binary(JSOP_ADD))
         return false;
 
-    if (current->setArg(GET_SLOTNO(pc)))
+    if (!current->setArg(GET_SLOTNO(pc)))
         return false;
 
     if (post_incr)
         current->pop();
 
     return true;
 }