diff --git a/src/Solcore/Frontend/TypeInference/TcStmt.hs b/src/Solcore/Frontend/TypeInference/TcStmt.hs
index 8cebd3ff..b07ba819 100644
--- a/src/Solcore/Frontend/TypeInference/TcStmt.hs
+++ b/src/Solcore/Frontend/TypeInference/TcStmt.hs
@@ -338,7 +338,7 @@ createClosureType ids vs ty
       i <- incCounter
       s <- getSubst
       let
-          ts = map idType ids 
+          ts = map idType ids
           dn = Name $ "t_closure" ++ show i
           ts' = everywhere (mkT gen) ts
           ns = map Var $ (apply s ids)
@@ -460,7 +460,7 @@ tiFunDef d@(FunDef sig@(Signature _ _ n args _) bd)
       -- typing function body
       (bd1, ps1, t1) <- withLocalCtx lctx' (tcBody bd) `wrapError` d
       -- unifying context introduced type with infered function type
-      s <- unify nt (funtype ts' t1) `wrapError` d 
+      s <- unify nt (funtype ts' t1) `wrapError` d
       -- building the function type scheme
       rs <- reduce [] ps1 `wrapError` d
       ty <- withCurrentSubst nt
@@ -515,15 +515,15 @@ tcFunDef incl vs' qs d@(FunDef sig@(Signature vs ps n args rt) bd)
       let lctx' = if incl then (n, monotype nt) : lctx else lctx
       -- typing function body
       (bd1', ps1', t1') <- withLocalCtx lctx' (tcBody bd1) `wrapError` d
-      -- checking if the type checking have changed the type 
+      -- checking if the type checking have changed the type
       -- due to unique type creation.
       let tynames = tyconNames t1'
-      changeTy <- or <$> mapM isUniqueTyName tynames 
+      changeTy <- or <$> mapM isUniqueTyName tynames
       let rt2 = if changeTy then t1' else rt1'
       info ["Trying to unify: ", pretty rt2, " with ", pretty t1']
       unify rt2 t1' `wrapError` d
       info ["Trying to unify: ", pretty nt, " with ", pretty (funtype ts' rt2)]
-      unify nt (funtype ts' rt2) `wrapError` d  
+      unify nt (funtype ts' rt2) `wrapError` d
       -- building the function type scheme
       free <- getEnvMetaVars
       rs <- reduce (qs1 `union` ps1) ps1' `wrapError` d
@@ -536,11 +536,11 @@ tcFunDef incl vs' qs d@(FunDef sig@(Signature vs ps n args rt) bd)
       when (ambiguous inf) $
         ambiguousTypeError inf sig
       -- checking subsumption
-      unless changeTy $ do 
+      unless changeTy $ do
         subsCheck sig inf ann `wrapError` d
       -- elaborating function body
       let ann' = if changeTy then inf else ann
-      fdt <- elabFunDef vs' sig1 bd1' inf ann' `wrapError` d 
+      fdt <- elabFunDef vs' sig1 bd1' inf ann' `wrapError` d
       withCurrentSubst (fdt, ann')
   | otherwise = tiFunDef d
 
@@ -691,7 +691,7 @@ verifySignatures instd@(Instance _ _ ps n ts t funs) =
   do
     -- get class info
     mcinfo <- Map.lookup n <$> gets classTable
-    when (isNothing mcinfo) (undefinedClass n) `wrapError` instd 
+    when (isNothing mcinfo) (undefinedClass n) `wrapError` instd
     -- building instance constraint
     let
       -- this use of fromJust is safe, because is
@@ -813,10 +813,10 @@ checkInstance idef@(Instance d vs ctx n ts t funs)
       -- checking the coverage condition
       insts' <- askInstEnv n `wrapError` ipred
       -- check overlapping only for non-default instances
-      let vs1 = bv ipred 
-      ts1 <- mapM (const freshTyVar) vs1 
+      let vs1 = bv ipred
+      ts1 <- mapM (const freshTyVar) vs1
       let env = zip vs1 ts1
-          ipred' = insts env ipred 
+          ipred' = insts env ipred
       unless d (checkOverlap ipred' insts' `wrapError` idef)
       -- check if default instance has a type variable as main argument.
       when d (checkDefaultInst (ctx :=> ipred) `wrapError` idef)
@@ -1071,13 +1071,14 @@ tcYulExp (YIdent v)
       (_ :=> t) <- freshInst sch
       unify t word
       pure t
-tcYulExp (YCall n es)
+tcYulExp e@(YCall n es)
   = do
-      sch <- askEnv n `wrapError` (YCall n es)
+      sch <- askEnv n `wrapError` e
       (_ :=> t) <- freshInst sch
       ts <- mapM tcYulExp es
       t' <- freshTyVar
-      unify t (foldr (:->) t' ts)
+      s <- unify t (funtype ts t') `wrapError` e
+      extSubst s
       withCurrentSubst t'
 
 tcYLit :: YLiteral -> TcM Ty
diff --git a/src/Solcore/Primitives/Primitives.hs b/src/Solcore/Primitives/Primitives.hs
index d72d6759..f99588e0 100644
--- a/src/Solcore/Primitives/Primitives.hs
+++ b/src/Solcore/Primitives/Primitives.hs
@@ -186,7 +186,7 @@ yulPrimOps = [ (Name "stop", monotype unit)
              , (Name "slt", monotype (word :-> word :-> word))
              , (Name "sgt", monotype (word :-> word :-> word))
              , (Name "eq", monotype (word :-> word :-> word))
-             , (Name "iszero", monotype (word :-> word :-> word))
+             , (Name "iszero", monotype (word :-> word))
              , (Name "and", monotype (word :-> word :-> word))
              , (Name "or", monotype (word :-> word :-> word))
              , (Name "xor", monotype (word :-> word :-> word))
diff --git a/test/Cases.hs b/test/Cases.hs
index 775f0a2a..0a9d3ac8 100644
--- a/test/Cases.hs
+++ b/test/Cases.hs
@@ -187,6 +187,7 @@ cases =
     , runTestExpectingFailure "overlapping-heads.solc" caseFolder
     , runTestExpectingFailure "instance-wrong-sig.solc" caseFolder
     , runTestForFile "match-yul.solc" caseFolder
+    , runTestForFile "yul-function-typing.solc" caseFolder
     ]
  where
   caseFolder = "./test/examples/cases"
diff --git a/test/examples/cases/yul-function-typing.solc b/test/examples/cases/yul-function-typing.solc
new file mode 100644
index 00000000..32812c24
--- /dev/null
+++ b/test/examples/cases/yul-function-typing.solc
@@ -0,0 +1,8 @@
+function foo(length:word, pos:word) -> word {
+  let ret: word;
+  assembly {
+	// ret := add(pos, mul(0x20, iszero(iszero(length))))
+	ret := iszero(iszero(length))
+  }
+  return ret;
+}