diff --git a/app/docs/user.md b/app/docs/user.md
index 1e71f8f..ac555d0 100644
--- a/app/docs/user.md
+++ b/app/docs/user.md
@@ -2,6 +2,8 @@
 
 This function is used to compare two number arrays/vectors/matrices, provided absolute and/or relative tolerance parameters `rtol` and `atol`. This is carried out using the [numpy.allclose](https://numpy.org/doc/stable/reference/generated/numpy.allclose.html) function.
 
+If the answer is not an array of numbers an exception is raised. If the response is not an array of numbers, a feedback message that informs the user that only numbers are accepte will be generated.
+
 ### Optional parameters
 
 There is one optional parameter: `feedback_for_incorrect_response`.
diff --git a/app/evaluation.py b/app/evaluation.py
index 6a14833..68669ed 100644
--- a/app/evaluation.py
+++ b/app/evaluation.py
@@ -23,29 +23,41 @@ def evaluation_function(response, answer, params) -> dict:
     to output the grading response.
     """
 
-    answer_ok = process_element(answer)
-    if not answer_ok:
+    try:
+        process_element(answer)
+    except ValueError:
+        raise Exception("Answer has at least one field that is not a number.")
+    except Exception:
         raise Exception("Answer has empty fields.")
-    response_ok = process_element(response)
-    if not response_ok:
+        return {"is_correct": False}
+
+    feedback = ""
+    try:
+        process_element(response)
+    except ValueError:
+        feedback = "Only numbers are permitted."
+    except Exception:
+        feedback = "Response has at least one empty field."
+
+    if len(feedback) > 0:
         return {
             "is_correct": False,
-            "feedback": "Response has empty fields."
+            "feedback": feedback,
         }
 
     try:
-        res = np.array(response, dtype=np.float32)
-    except Exception as e:
+        ans = np.array(answer, dtype=np.float32)
+    except ValueError as e:
         raise EvaluationException(
-            f"Failed to parse user response",
+            f"Failed to parse correct answer",
             detail=repr(e)
         )
 
     try:
-        ans = np.array(answer, dtype=np.float32)
-    except Exception as e:
+        res = np.array(response, dtype=np.float32)
+    except ValueError as e:
         raise EvaluationException(
-            f"Failed to parse correct answer",
+            f"Failed to parse response",
             detail=repr(e)
         )
 
@@ -65,14 +77,14 @@ def evaluation_function(response, answer, params) -> dict:
 
 def process_element(element):
     is_ok = True
-    if isinstance(element,list):
+    if isinstance(element, list):
         for e in element:
-            is_ok = process_element(e)
+            process_element(e)
     else:
-        if isinstance(element,str):
+        if isinstance(element, str):
             element = element.strip()
             if len(element) == 0 or "element" == "undefined":
-                is_ok = False
+                raise Exception("Contains an empty element")
             else:
                 element = float(element)
-    return is_ok
\ No newline at end of file
+    return
\ No newline at end of file
diff --git a/app/evaluation_tests.py b/app/evaluation_tests.py
index ee26ec2..0a68fe8 100644
--- a/app/evaluation_tests.py
+++ b/app/evaluation_tests.py
@@ -44,7 +44,7 @@ def test_2D_empty_string_in_response(self):
         response = evaluation_function(response, answer, {})
 
         self.assertEqual(response["is_correct"], False)
-        self.assertEqual(response["feedback"], "Response has empty fields.")
+        self.assertEqual(response["feedback"], "Response has at least one empty field.")
 
     def test_no_tolerance_correct(self):
         response = [1, 2]
@@ -132,6 +132,26 @@ def test_2D_incorrect_with_custom_feedback(self):
 #
 #        self.assertEqual(response.get("is_correct"), True)
 
+    def test_answer_not_array_of_numbers(self):
+        response = [[1, 1], [1, 1]]
+        answer = [[1, 1], [1, "a"]]
+
+        self.assertRaises(
+            Exception,
+            evaluation_function,
+            response,
+            answer,
+            {},
+        )
+
+    def test_response_not_array_of_numbers(self):
+        response = [[1, 1], [1, "a"]]
+        answer = [[1, 1], [1, 0]]
+
+        response = evaluation_function(response, answer, {})
+
+        self.assertEqual(response.get("is_correct"), False)
+        self.assertEqual(response["feedback"], "Only numbers are permitted.")
 
 if __name__ == "__main__":
     unittest.main()