diff --git a/proof_checker.py b/proof_checker.py
index 7980734..ec3de60 100644
--- a/proof_checker.py
+++ b/proof_checker.py
@@ -2382,6 +2382,35 @@ def is_modified(filename):
     else:
         return True
           
+def validate_union_type(ty, params, env):
+  match ty:
+    case Var(loc, t, name, rns):
+      type_def = env.get_def_of_type_var(ty)
+
+      if isinstance(type_def, Union):
+        union_params_len = len(type_def.type_params)
+        provided_params_len = len(params)
+        if union_params_len != provided_params_len:
+          error(ty.location, f"Expected union type '{ty}' in constructor parameters " \
+               + f"to have {union_params_len} parameters, not {provided_params_len}")
+    case TypeInst(loc, ty, type_args):
+      for t in type_args:
+        validate_union_type(t, [], env)
+      validate_union_type(ty, type_args, env)
+      pass
+    case FunctionType(loc, ty_params, param_types, return_type):  
+      for t in param_types:
+        validate_union_type(t, [], env)
+      validate_union_type(return_type, [], env)
+    case IntType():
+      pass
+    case BoolType():
+      pass
+    case ArrayType(loc, elt_ty):
+      validate_union_type(elt_ty, [], env)
+    case _:
+      error(ty.location, f"Unhandled case '{type(ty)}' in 'validate_union_type")
+
 def process_declaration_visibility(decl : Declaration, env: Env, module_chain, downstream_needs_checking):
   match decl:
     case Define(loc, name, ty, body):
@@ -2459,6 +2488,7 @@ def process_declaration_visibility(decl : Declaration, env: Env, module_chain, d
             return_type = body_union_type
           for ty in constr.parameters:
             check_type(ty, body_env)
+            validate_union_type(ty, [], body_env)
           constr_type = FunctionType(constr.location, typarams,
                                      constr.parameters, return_type)
         elif len(typarams) > 0:
diff --git a/test/should-error/union_missing_typarams.pf b/test/should-error/union_missing_typarams.pf
new file mode 100644
index 0000000..229349b
--- /dev/null
+++ b/test/should-error/union_missing_typarams.pf
@@ -0,0 +1,9 @@
+union A<E> {
+    a(E)
+}
+
+union No<K, V> {
+    nil
+    th(No<K>)
+    thi(A)
+}
diff --git a/test/should-error/union_missing_typarams.pf.err b/test/should-error/union_missing_typarams.pf.err
new file mode 100644
index 0000000..b677e12
--- /dev/null
+++ b/test/should-error/union_missing_typarams.pf.err
@@ -0,0 +1 @@
+./test/should-error/union_missing_typarams.pf:7.8-7.10: Expected union type 'No' in constructor parameters to have 2 parameters, not 1
diff --git a/test/should-error/union_missing_typarams_fun.pf b/test/should-error/union_missing_typarams_fun.pf
new file mode 100644
index 0000000..028492c
--- /dev/null
+++ b/test/should-error/union_missing_typarams_fun.pf
@@ -0,0 +1,7 @@
+union A<E> {
+    a(E)
+}
+
+union No<K, V> {
+    thi(fn (A) -> bool)
+}
diff --git a/test/should-error/union_missing_typarams_fun.pf.err b/test/should-error/union_missing_typarams_fun.pf.err
new file mode 100644
index 0000000..6822b85
--- /dev/null
+++ b/test/should-error/union_missing_typarams_fun.pf.err
@@ -0,0 +1 @@
+./test/should-error/union_missing_typarams_fun.pf:6.13-6.14: Expected union type 'A' in constructor parameters to have 1 parameters, not 0
diff --git a/test/should-error/union_missing_typarams_rec.pf b/test/should-error/union_missing_typarams_rec.pf
new file mode 100644
index 0000000..1652b85
--- /dev/null
+++ b/test/should-error/union_missing_typarams_rec.pf
@@ -0,0 +1,9 @@
+union A<E> {
+    a(E)
+}
+
+union No<K, V> {
+    nil
+    th(No<K, V>)
+    thi(A)
+}
diff --git a/test/should-error/union_missing_typarams_rec.pf.err b/test/should-error/union_missing_typarams_rec.pf.err
new file mode 100644
index 0000000..9ead3c5
--- /dev/null
+++ b/test/should-error/union_missing_typarams_rec.pf.err
@@ -0,0 +1 @@
+./test/should-error/union_missing_typarams_rec.pf:8.9-8.10: Expected union type 'A' in constructor parameters to have 1 parameters, not 0