diff --git a/phasm/type3/constraints.py b/phasm/type3/constraints.py
index e04e66c..c5bf372 100644
--- a/phasm/type3/constraints.py
+++ b/phasm/type3/constraints.py
@@ -50,7 +50,7 @@ NewConstraintList = List['ConstraintBase']
 
 CheckResult = Union[None, SubstitutionMap, Error, NewConstraintList, RequireTypeSubstitutes]
 
-HumanReadableRet = Tuple[str, Dict[str, Union[str, ourlang.Expression, Type3, PlaceholderForType]]]
+HumanReadableRet = Tuple[str, Dict[str, Union[None, int, str, ourlang.Expression, Type3, PlaceholderForType]]]
 
 class Context:
     """
@@ -500,45 +500,42 @@ class CanBeSubscriptedConstraint(ConstraintBase):
     """
     A value that is subscipted, i.e. a[0] (tuple) or a[b] (static array)
     """
-    __slots__ = ('ret_type3', 'type3', 'index', 'index_phft', )
+    __slots__ = ('ret_type3', 'type3', 'index_type3', 'index_const', )
 
-    ret_type3: Type3OrPlaceholder
-    type3: Type3OrPlaceholder
-    index: ourlang.Expression
-    index_phft: Type3OrPlaceholder
+    ret_type3: PlaceholderForType
+    type3: PlaceholderForType
+    index_type3: PlaceholderForType
+    index_const: int | None
 
     def __init__(
             self,
             ret_type3: PlaceholderForType,
             type3: PlaceholderForType,
-            index: ourlang.Expression,
-            index_phft: PlaceholderForType,
+            index_type3: PlaceholderForType,
+            index_const: int | None,
             comment: Optional[str] = None,
         ) -> None:
         super().__init__(comment=comment)
 
         self.ret_type3 = ret_type3
         self.type3 = type3
-        self.index = index
-        self.index_phft = index_phft
+        self.index_type3 = index_type3
+        self.index_const = index_const
 
     def _generate_bytes(self) -> CheckResult:
         return [
-            SameTypeConstraint(prelude.u32, self.index_phft, comment='([]) :: bytes -> u32 -> u8'),
+            SameTypeConstraint(prelude.u32, self.index_type3, comment='([]) :: bytes -> u32 -> u8'),
             SameTypeConstraint(prelude.u8, self.ret_type3, comment='([]) :: bytes -> u32 -> u8'),
         ]
 
     def _generate_static_array(self, sa_args: tuple[Type3, IntType3]) -> CheckResult:
         sa_type, sa_len = sa_args
 
-        if isinstance(self.index, ourlang.ConstantPrimitive):
-            assert isinstance(self.index.value, int)
-
-            if self.index.value < 0 or sa_len.value <= self.index.value:
-                return Error('Tuple index out of range')
+        if self.index_const is not None and (self.index_const < 0 or sa_len.value <= self.index_const):
+            return Error('Tuple index out of range')
 
         return [
-            SameTypeConstraint(prelude.u32, self.index_phft, comment='([]) :: Subscriptable a => a b -> u32 -> b'),
+            SameTypeConstraint(prelude.u32, self.index_type3, comment='([]) :: Subscriptable a => a b -> u32 -> b'),
             SameTypeConstraint(sa_type, self.ret_type3, comment='([]) :: Subscriptable a => a b -> u32 -> b'),
         ]
 
@@ -547,18 +544,15 @@ class CanBeSubscriptedConstraint(ConstraintBase):
         # e.g. rather than having to do `fst a` and `snd a` and only have to-sized tuples
         # we use a[0] and a[1] and allow for a[2] and on.
 
-        if not isinstance(self.index, ourlang.ConstantPrimitive):
-            return Error('Must index with literal')
-
-        if not isinstance(self.index.value, int):
+        if self.index_const is None:
             return Error('Must index with integer literal')
 
-        if self.index.value < 0 or len(tp_args) <= self.index.value:
+        if self.index_const < 0 or len(tp_args) <= self.index_const:
             return Error('Tuple index out of range')
 
         return [
-            SameTypeConstraint(prelude.u32, self.index_phft, comment=f'Tuple subscript index {self.index.value}'),
-            SameTypeConstraint(tp_args[self.index.value], self.ret_type3, comment=f'Tuple subscript index {self.index.value}'),
+            SameTypeConstraint(prelude.u32, self.index_type3, comment='([]) :: Subscriptable a => a b -> u32 -> b'),
+            SameTypeConstraint(tp_args[self.index_const], self.ret_type3, comment=f'Tuple subscript index {self.index_const}'),
         ]
 
     GENERATE_ROUTER = TypeApplicationRouter['CanBeSubscriptedConstraint', CheckResult]()
@@ -567,12 +561,10 @@ class CanBeSubscriptedConstraint(ConstraintBase):
     GENERATE_ROUTER.add(prelude.tuple_, _generate_tuple)
 
     def check(self) -> CheckResult:
-        exp_type = self.type3
-        if isinstance(exp_type, PlaceholderForType):
-            if exp_type.resolve_as is None:
-                return RequireTypeSubstitutes()
+        if self.type3.resolve_as is None:
+            return RequireTypeSubstitutes()
 
-            exp_type = exp_type.resolve_as
+        exp_type = self.type3.resolve_as
 
         try:
             return self.__class__.GENERATE_ROUTER(self, exp_type)
@@ -584,9 +576,9 @@ class CanBeSubscriptedConstraint(ConstraintBase):
             '{type3}[{index}]',
             {
                 'type3': self.type3,
-                'index': self.index,
+                'index': self.index_type3 if self.index_const is None else self.index_const,
             },
         )
 
     def __repr__(self) -> str:
-        return f'CanBeSubscriptedConstraint({repr(self.type3)}, {repr(self.index)}, comment={repr(self.comment)})'
+        return f'CanBeSubscriptedConstraint({self.ret_type3!r}, {self.type3!r}, {self.index_type3!r}, {self.index_const!r}, comment={repr(self.comment)})'
diff --git a/phasm/type3/constraintsgenerator.py b/phasm/type3/constraintsgenerator.py
index c956918..01c1549 100644
--- a/phasm/type3/constraintsgenerator.py
+++ b/phasm/type3/constraintsgenerator.py
@@ -193,7 +193,10 @@ def expression(ctx: Context, inp: ourlang.Expression, phft: PlaceholderForType)
         yield from expression(ctx, inp.varref, varref_phft)
         yield from expression(ctx, inp.index, index_phft)
 
-        yield CanBeSubscriptedConstraint(phft, varref_phft, inp.index, index_phft)
+        if isinstance(inp.index, ourlang.ConstantPrimitive) and isinstance(inp.index.value, int):
+            yield CanBeSubscriptedConstraint(phft, varref_phft, index_phft, inp.index.value)
+        else:
+            yield CanBeSubscriptedConstraint(phft, varref_phft, index_phft, None)
         return
 
     if isinstance(inp, ourlang.AccessStructMember):
diff --git a/tests/integration/test_lang/test_subscriptable.py b/tests/integration/test_lang/test_subscriptable.py
index 1009227..b6b0d35 100644
--- a/tests/integration/test_lang/test_subscriptable.py
+++ b/tests/integration/test_lang/test_subscriptable.py
@@ -64,7 +64,7 @@ def testEntry(x: (u8, u32, u64), y: u8) -> u64:
     return x[y]
 """
 
-    with pytest.raises(Type3Exception, match='Must index with literal'):
+    with pytest.raises(Type3Exception, match='Must index with integer literal'):
         Suite(code_py).run_code()
 
 @pytest.mark.integration_test