diff --git a/backends/GenerateC.hs b/backends/GenerateC.hs
index d9c5efd..6b7f0af 100644
--- a/backends/GenerateC.hs
+++ b/backends/GenerateC.hs
@@ -904,6 +904,10 @@ cgenExpression (A.BytesInType m t) = call genBytesIn m t (Left False)
 --cgenExpression (A.ExprConstr {})
 cgenExpression (A.AllocMobile m t me) = call genAllocMobile m t me
 cgenExpression (A.CloneMobile m e) = call genCloneMobile m e
+cgenExpression (A.IsDefined m (A.ExprVariable _ (A.DerefVariable _ v)))
+  = tell ["("] >> call genVariable v A.Original >> tell ["!=NULL)"]
+cgenExpression (A.IsDefined m e)
+  = tell ["("] >> call genExpression e >> tell ["!=NULL)"]
 cgenExpression (A.SubscriptedExpr m sub (A.ExprVariable _ v))
   = call genVariable (A.SubscriptedVariable m sub v) A.Original
 cgenExpression (A.SubscriptedExpr m (A.SubscriptFromFor _ _ start _) e@(A.AllSizesVariable {}))
diff --git a/transformations/SimplifyExprs.hs b/transformations/SimplifyExprs.hs
index 1593ed6..9379714 100644
--- a/transformations/SimplifyExprs.hs
+++ b/transformations/SimplifyExprs.hs
@@ -399,6 +399,8 @@ pullUp pullUpArraysInsideRecords = pass "Pull up definitions"
     -- | Pull array expressions that aren't already non-subscripted variables.
     -- Also pull lists that are literals or constructed
     doExpression :: A.Expression -> PassM A.Expression
+    -- For is-defined, we don't want to pull up:
+    doExpression e@(A.IsDefined {}) = return e
     doExpression e
               -- This part handles recursing into the expression first:
         =  do e' <- doExpression' e