diff --git a/lib/cretonne/meta/cdsl/instructions.py b/lib/cretonne/meta/cdsl/instructions.py
index 6dec0b7dff..30c27c6649 100644
--- a/lib/cretonne/meta/cdsl/instructions.py
+++ b/lib/cretonne/meta/cdsl/instructions.py
@@ -267,6 +267,16 @@ class Instruction(object):
 
         return other_tvs
 
+    def all_typevars(self):
+        # type: () -> List[TypeVar]
+        """
+        Get a list of all type variables in the instruction.
+        """
+        if self.is_polymorphic:
+            return [self.ctrl_typevar] + self.other_typevars
+        else:
+            return []
+
     @staticmethod
     def _to_operand_tuple(x):
         # type: (Union[Sequence[Operand], Operand]) -> Tuple[Operand, ...]
diff --git a/lib/cretonne/meta/cdsl/ti.py b/lib/cretonne/meta/cdsl/ti.py
index 028579857d..b2673366be 100644
--- a/lib/cretonne/meta/cdsl/ti.py
+++ b/lib/cretonne/meta/cdsl/ti.py
@@ -796,11 +796,7 @@ def ti_def(definition, typ):
 
     # Create a dict m mapping each free typevar in the signature of definition
     # to a fresh copy of itself.
-    if inst.is_polymorphic:
-        free_formal_tvs = [inst.ctrl_typevar] + inst.other_typevars
-    else:
-        free_formal_tvs = []
-
+    free_formal_tvs = inst.all_typevars()
     m = {tv: tv.get_fresh_copy(str(typ.get_uid())) for tv in free_formal_tvs}
 
     # Update m with any explicitly bound type vars