diff --git a/src/builder/build_traits.rs b/src/builder/build_traits.rs
index 8c314294d..8f5341b08 100644
--- a/src/builder/build_traits.rs
+++ b/src/builder/build_traits.rs
@@ -20,7 +20,7 @@ use crate::{
 };
 
 use crate::extension::{ExtensionRegistry, ExtensionSet, PRELUDE_REGISTRY};
-use crate::types::{FunctionType, PolyFuncType, Type, TypeRow};
+use crate::types::{FunctionType, PolyFuncType, Type, TypeArg, TypeRow};
 
 use itertools::Itertools;
 
@@ -595,7 +595,9 @@ pub trait Dataflow: Container {
     fn call<const DEFINED: bool>(
         &mut self,
         function: &FuncID<DEFINED>,
+        type_args: &[TypeArg],
         input_wires: impl IntoIterator<Item = Wire>,
+        exts: &ExtensionRegistry, // TODO remove?
     ) -> Result<BuildHandle<DataflowOpID>, BuildError> {
         let hugr = self.hugr();
         let def_op = hugr.get_optype(function.node());
@@ -609,9 +611,16 @@ pub trait Dataflow: Container {
                 })
             }
         };
-        let signature = type_scheme.body;
+        // TODO either some way of returning a SignatureError without a node (as its not
+        // constructed yet) - or, preferably, a way to "instantiate" the PolyFuncType without
+        // validating against an ExtensionRegistry.
+        let signature = type_scheme.instantiate(type_args, exts).map_err(|e| {
+            BuildError::InvalidHUGR(ValidationError::SignatureError {
+                node: function.node(),
+                cause: e,
+            })
+        })?;
         let const_in_port = signature.output.len();
-        // TODO ALAN this is totally broken for polymorphic functions. Need to TypeApply here.
         let op_id = self.add_dataflow_op(ops::Call { signature }, input_wires)?;
         let src_port = self.hugr_mut().num_outputs(function.node()) - 1;
 
diff --git a/src/builder/module.rs b/src/builder/module.rs
index c3bb1c3d5..92b9e396a 100644
--- a/src/builder/module.rs
+++ b/src/builder/module.rs
@@ -169,7 +169,7 @@ mod test {
             test::{n_identity, NAT},
             Dataflow, DataflowSubContainer,
         },
-        extension::EMPTY_REG,
+        extension::{EMPTY_REG, PRELUDE_REGISTRY},
         type_row,
         types::FunctionType,
     };
@@ -186,7 +186,7 @@ mod test {
             )?;
 
             let mut f_build = module_builder.define_declaration(&f_id)?;
-            let call = f_build.call(&f_id, f_build.input_wires())?;
+            let call = f_build.call(&f_id, &[], f_build.input_wires(), &PRELUDE_REGISTRY)?;
 
             f_build.finish_with_outputs(call.outputs())?;
             module_builder.finish_prelude_hugr()
@@ -234,7 +234,8 @@ mod test {
             let [wire] = local_build.input_wires_arr();
             let f_id = local_build.finish_with_outputs([wire])?;
 
-            let call = f_build.call(f_id.handle(), f_build.input_wires())?;
+            let call =
+                f_build.call(f_id.handle(), &[], f_build.input_wires(), &PRELUDE_REGISTRY)?;
 
             f_build.finish_with_outputs(call.outputs())?;
             module_builder.finish_prelude_hugr()