diff --git a/lib/cretonne/meta/cdsl/ti.py b/lib/cretonne/meta/cdsl/ti.py
index 7a95daf425..028579857d 100644
--- a/lib/cretonne/meta/cdsl/ti.py
+++ b/lib/cretonne/meta/cdsl/ti.py
@@ -8,12 +8,13 @@ from itertools import product
 
 try:
     from typing import Dict, TYPE_CHECKING, Union, Tuple, Optional, Set # noqa
-    from typing import Iterable, List, Any # noqa
+    from typing import Iterable, List, Any, TypeVar as MTypeVar # noqa
     from typing import cast
     from .xform import Rtl, XForm # noqa
     from .ast import Expr # noqa
     from .typevar import TypeSet # noqa
     if TYPE_CHECKING:
+        T = MTypeVar('T')
         TypeMap = Dict[TypeVar, TypeVar]
         VarTyping = Dict[Var, TypeVar]
 except ImportError:
@@ -775,6 +776,11 @@ def unify(tv1, tv2, typ):
     return typ
 
 
+def move_first(l, i):
+    # type: (List[T], int) -> List[T]
+    return [l[i]] + l[:i] + l[i+1:]
+
+
 def ti_def(definition, typ):
     # type: (Def, TypeEnv) -> TypingOrError
     """
@@ -821,6 +827,12 @@ def ti_def(definition, typ):
         typ.register(v)
         actual_tvs.append(v.get_typevar())
 
+    # Make sure we unify the control typevar first.
+    if inst.is_polymorphic:
+        idx = fresh_formal_tvs.index(m[inst.ctrl_typevar])
+        fresh_formal_tvs = move_first(fresh_formal_tvs, idx)
+        actual_tvs = move_first(actual_tvs, idx)
+
     # Unify each actual typevar with the correpsonding fresh formal tv
     for (actual_tv, formal_tv) in zip(actual_tvs, fresh_formal_tvs):
         typ_or_err = unify(actual_tv, formal_tv, typ)