diff --git a/midas/checker/unifier.py b/midas/checker/unifier.py
index e414623..aae9310 100644
--- a/midas/checker/unifier.py
+++ b/midas/checker/unifier.py
@@ -12,6 +12,9 @@ from midas.checker.types import (
 )
 
 
+class UnificationError(Exception): ...
+
+
 class Unifier:
     def __init__(self, types: TypesRegistry) -> None:
         self.types: TypesRegistry = types
@@ -45,10 +48,20 @@ class Unifier:
             ],
             returns=TopType(),  # TODO: use expected type
         )
-        return self.unify_generic(type, concrete_func)
+        return self.unify_generic(type, concrete_func, match_return=False)
+
+    def unify_generic(
+        self,
+        template: GenericType,
+        concrete: Type,
+        match_return: bool = True,
+    ) -> Optional[Type]:
+        substitutions: dict[str, Type]
+        try:
+            substitutions = self.match(template.body, concrete, match_return)
+        except UnificationError:
+            return None
 
-    def unify_generic(self, template: GenericType, concrete: Type) -> Optional[Type]:
-        substitutions: dict[str, Type] = self.match(template.body, concrete)
         args: list[Type] = []
         for param in template.params:
             if param.name not in substitutions:
@@ -58,7 +71,12 @@ class Unifier:
         applied: Type = self.types.apply_generic(template, args)
         return applied
 
-    def match(self, template: Type, concrete: Type) -> dict[str, Type]:
+    def match(
+        self,
+        template: Type,
+        concrete: Type,
+        match_return: bool = True,
+    ) -> dict[str, Type]:
         # TODO: if concrete is Generic, record bound TypeVar. Then when merging
         # substitutions, check that the constraint is respected
         match (template, concrete):
@@ -91,10 +109,11 @@ class Unifier:
                     )
                     substitutions = self.merge(substitutions, arg_subs)
 
-                return_subs: dict[str, Type] = self.match(
-                    template.returns, concrete.returns
-                )
-                substitutions = self.merge(substitutions, return_subs)
+                if match_return:
+                    return_subs: dict[str, Type] = self.match(
+                        template.returns, concrete.returns
+                    )
+                    substitutions = self.merge(substitutions, return_subs)
 
                 return substitutions
 
@@ -110,6 +129,7 @@ class Unifier:
                 self.logger.debug(
                     f"Substitution already defined for {k} with type {merged[k]}, got {v}"
                 )
+                raise UnificationError
             merged[k] = v
         return merged
 
diff --git a/tests/cases/checker/08_unification.py.ref.json b/tests/cases/checker/08_unification.py.ref.json
index fd36cbd..bfaa7bd 100644
--- a/tests/cases/checker/08_unification.py.ref.json
+++ b/tests/cases/checker/08_unification.py.ref.json
@@ -1,5 +1,20 @@
 {
-  "diagnostics": [],
+  "diagnostics": [
+    {
+      "type": "Error",
+      "location": {
+        "start": [
+          13,
+          15
+        ],
+        "end": [
+          13,
+          32
+        ]
+      },
+      "message": "Could not unify map[T, U]=(transform: (v: T, /) -> U, iterable: list[T], /) -> list[U] with pos=[(value: float) -> float, list[int]] and kw={}"
+    }
+  ],
   "judgments": [
     {
       "location": {
@@ -682,17 +697,7 @@
         ],
         "keywords": {}
       },
-      "type": {
-        "name": "list",
-        "args": [
-          {
-            "name": "float"
-          }
-        ],
-        "body": {
-          "name": "list"
-        }
-      }
+      "type": {}
     },
     {
       "location": {