diff --git a/app/Dockerfile b/app/Dockerfile
index 7832b6e..8b8ef7b 100644
--- a/app/Dockerfile
+++ b/app/Dockerfile
@@ -10,6 +10,7 @@ RUN pip3 install -r requirements.txt
 
 # Copy the evaluation and testing scripts
 COPY evaluation.py ./app/
+COPY shape.py ./app/
 COPY evaluation_test.py ./app/
 
 # Copy Documentation
diff --git a/app/docs/user.md b/app/docs/user.md
index 9580963..624d45b 100644
--- a/app/docs/user.md
+++ b/app/docs/user.md
@@ -3,4 +3,7 @@
 This function compares two symbolic expression arrays, with any level of nesting (2D, 3D, irregular shape, ...). Each cell is compared using the [`SymbolicEqual`](https://lambda-feedback.github.io/user-documentation/user_eval_function_docs/symbolicEqual/) evaluation function.
 
 ## Inputs
-This function support all input parameters that [`SymbolicEqual`](https://lambda-feedback.github.io/user-documentation/user_eval_function_docs/symbolicEqual/) uses.
\ No newline at end of file
+This function support all input parameters that [`SymbolicEqual`](https://lambda-feedback.github.io/user-documentation/user_eval_function_docs/symbolicEqual/) uses.
+
+## Size mismatches
+If the response does not have the same size or shape as the answer (e.g. too many or too few entries, or a list where a single expression was expected), the response is marked incorrect and the feedback explains the difference, e.g. "Your response has 2 rows and 3 columns, but the expected answer has 3 rows and 2 columns." No cells are graded in this case.
diff --git a/app/evaluation.py b/app/evaluation.py
index feb5a7f..551b8b2 100644
--- a/app/evaluation.py
+++ b/app/evaluation.py
@@ -1,9 +1,13 @@
-import numpy as np
 import re
 
 from evaluation_function_utils.client import EvaluationFunctionClient
 from evaluation_function_utils.errors import EvaluationException
 
+try:
+    from .shape import shape_feedback
+except ImportError:
+    from shape import shape_feedback
+
 client = EvaluationFunctionClient(env_path=".env")
 
 
@@ -79,11 +83,19 @@ def evaluation_function(response, answer, params):
     to output the grading response.
     """
 
-    if not (isinstance(response, list) and isinstance(answer, list)):
+    if not isinstance(answer, list):
         raise EvaluationException(
-            f"Response, Answer given of type {type(response)}, {type(answer)}: types unsupported"
+            f"Answer given of type {type(answer)}: type unsupported"
         )
 
+    # Give feedback on a size/shape mismatch rather than failing during grading
+    size_feedback = shape_feedback(response, answer)
+    if size_feedback is not None:
+        return {
+            "is_correct": False,
+            "feedback": size_feedback
+        }
+
     # Grade all cells recursively (to allow any shape input)
     detailed_feedback, feedback = recursive_grade(params, response, answer, [],
                                                   [], [])
diff --git a/app/evaluation_test.py b/app/evaluation_test.py
index 4c045d5..eab13ae 100644
--- a/app/evaluation_test.py
+++ b/app/evaluation_test.py
@@ -1,13 +1,26 @@
 import unittest
+from unittest import mock
 
 from evaluation_function_utils.client import MissingCredentials
 
+
+def import_evaluation():
+    try:
+        from . import evaluation
+    except ImportError:
+        import evaluation
+    return evaluation
+
+
+# The real client needs AWS credentials on creation. Without them, import
+# with a mock client so the tests below (which mock the client) can still run.
 try:
-    from .evaluation import evaluation_function
-except ImportError:
-    from evaluation import evaluation_function
+    evaluation = import_evaluation()
 except MissingCredentials:
-    pass
+    with mock.patch("evaluation_function_utils.client.EvaluationFunctionClient"):
+        evaluation = import_evaluation()
+
+evaluation_function = evaluation.evaluation_function
 
 
 class TestEvaluationFunction(unittest.TestCase):
@@ -29,8 +42,73 @@ class TestEvaluationFunction(unittest.TestCase):
         as it should.
     """
 
-    def test_blank(self):
-        self.assertTrue(True)
+    def setUp(self):
+        patcher = mock.patch.object(evaluation, "client")
+        self.client = patcher.start()
+        self.addCleanup(patcher.stop)
+
+    def assert_size_feedback(self, response, answer, feedback):
+        result = evaluation_function(response, answer, {})
+
+        self.assertEqual(result.get("is_correct"), False)
+        self.assertEqual(result.get("feedback"), feedback)
+        # Size mismatches are reported before any cell is graded
+        self.client.invoke.assert_not_called()
+
+    def test_vector_too_long(self):
+        self.assert_size_feedback(
+            ["a", "b", "c"], ["a", "b"],
+            "Your response has 3 entries, but the expected answer has 2 entries.")
+
+    def test_vector_too_short(self):
+        self.assert_size_feedback(
+            ["a"], ["a", "b"],
+            "Your response has 1 entry, but the expected answer has 2 entries.")
+
+    def test_matrix_wrong_size(self):
+        self.assert_size_feedback(
+            [["a", "b", "c"], ["d", "e", "f"]], [["a", "b"], ["c", "d"], ["e", "f"]],
+            "Your response has 2 rows and 3 columns, but the expected answer has 3 rows and 2 columns.")
+
+    def test_vector_for_matrix(self):
+        self.assert_size_feedback(
+            ["a", "b", "c", "d"], [["a", "b"], ["c", "d"]],
+            "Your response has 4 entries, but the expected answer has 2 rows and 2 columns.")
+
+    def test_list_for_single_expression(self):
+        self.assert_size_feedback(
+            ["a", ["b"]], ["a", "b"],
+            "Entry (2) should be a single expression, not a list.")
+
+    def test_single_expression_for_list(self):
+        self.assert_size_feedback(
+            "a", ["a", "b"],
+            "Your response should be an array of 2 entries.")
+
+    def test_irregular_mismatch(self):
+        self.assert_size_feedback(
+            [["a"], ["b", "c", "d"]], [["a"], ["b", "c"]],
+            "Entry (2) has 3 entries, but should have 2 entries.")
+
+    def test_same_size_is_graded(self):
+        self.client.invoke.return_value = {"is_correct": True}
+
+        result = evaluation_function([["a", "b"], ["c", "d"]], [["a", "b"], ["c", "d"]], {})
+
+        self.assertEqual(result.get("is_correct"), True)
+        self.assertEqual(self.client.invoke.call_count, 4)
+
+    def test_same_shape_different_values_is_graded(self):
+        self.client.invoke.return_value = {"is_correct": True}
+
+        result = evaluation_function([["a+b", "c"], ["d", "e"]], [["b+a", "c"], ["d", "e"]], {})
+
+        self.assertEqual(result.get("is_correct"), True)
+        self.assertEqual(self.client.invoke.call_count, 4)
+
+    def test_answer_not_list(self):
+        with self.assertRaises(evaluation.EvaluationException):
+            evaluation_function(["a"], "a", {})
 
     """
     NOTE: This function was converted to use the experimental
diff --git a/app/shape.py b/app/shape.py
new file mode 100644
index 0000000..90555b8
--- /dev/null
+++ b/app/shape.py
@@ -0,0 +1,77 @@
+import numpy as np
+
+
+def plural(n, word, word_plural=None):
+    if n == 1:
+        return f"{n} {word}"
+    return f"{n} {word_plural or word + 's'}"
+
+
+def entries(n):
+    return plural(n, "entry", "entries")
+
+
+def describe_shape(shape):
+    if len(shape) == 1:
+        return entries(shape[0])
+    if len(shape) == 2:
+        return f"{plural(shape[0], 'row')} and {plural(shape[1], 'column')}"
+    return "size " + " x ".join(str(n) for n in shape)
+
+
+def array_shape(array):
+    """
+    Return the shape of a regular (rectangular) array, e.g. (2, 3) for a
+    2x3 matrix, or None if the array is ragged.
+    """
+    try:
+        return np.shape(array)
+    except ValueError:
+        return None
+
+
+def entry_lengths(array):
+    """
+    Return the length of each top-level entry, with -1 for an entry that
+    is a single expression rather than a list.
+    """
+    return np.array([len(e) if isinstance(e, list) else -1 for e in array])
+
+
+def shape_feedback(response, answer):
+    """
+    Return feedback explaining how the size of the response differs
+    from the answer, or None if they have the same shape.
+    """
+    if not isinstance(response, list):
+        return f"Your response should be an array of {entries(len(answer))}."
+
+    response_shape = array_shape(response)
+    answer_shape = array_shape(answer)
+
+    if response_shape is not None and answer_shape is not None:
+        if response_shape == answer_shape:
+            return None
+        return f"Your response has {describe_shape(response_shape)}, "\
+               f"but the expected answer has {describe_shape(answer_shape)}."
+
+    # At least one of the arrays is ragged, so numpy can't give its shape
+    if len(response) != len(answer):
+        return f"Your response has {entries(len(response))}, "\
+               f"but the expected answer has {entries(len(answer))}."
+
+    mismatches = np.argwhere(entry_lengths(response) != entry_lengths(answer)).ravel()
+    if len(mismatches) == 0:
+        # The mismatch is more than one level deep
+        return "Your response does not have the same shape as the expected answer."
+
+    i = mismatches[0]
+    response_length = int(entry_lengths(response)[i])
+    answer_length = int(entry_lengths(answer)[i])
+    where = f"Entry ({i + 1})"
+
+    if answer_length == -1:
+        return f"{where} should be a single expression, not a list."
+    if response_length == -1:
+        return f"{where} should be an array of {entries(answer_length)}."
+    return f"{where} has {entries(response_length)}, but should have {entries(answer_length)}."