diff --git a/src/mind_the_gaps/gaps.py b/src/mind_the_gaps/gaps.py
index 8e81c33..fc91b87 100644
--- a/src/mind_the_gaps/gaps.py
+++ b/src/mind_the_gaps/gaps.py
@@ -178,6 +178,10 @@ def __post_init__(self):
         for i, endpoint in enumerate(self.endpoints):
             if not isinstance(endpoint, Endpoint):
                 self.endpoints[i] = Endpoint(endpoint, "[" if i % 2 == 0 else "]")
+            elif i % 2 == 0 and endpoint.boundary in ")]":
+                raise ValueError(f"Expected left boundary, got {endpoint!r}.")
+            elif i % 2 == 1 and endpoint.boundary in "([":
+                raise ValueError(f"Expected right boundary, got {endpoint!r}.")
 
         for i in range(len(self.endpoints) - 1):
             a = self.endpoints[i]
@@ -186,7 +190,7 @@ def __post_init__(self):
                 raise ValueError("Intervals unsorted.")
             if a.value == b.value and a.boundary + b.boundary in {"](", ")["}:
                 raise ValueError(
-                    f"Intervals not minimally expressed. Endpoints {a} and {b} can be removed."
+                    f"Intervals not minimally expressed. Endpoints {a!r} and {b!r} can be removed."
                 )
 
     @classmethod
diff --git a/tests/test_gap_init.py b/tests/test_gap_init.py
index d7998f9..a03bbf0 100644
--- a/tests/test_gap_init.py
+++ b/tests/test_gap_init.py
@@ -2,6 +2,14 @@
 from mind_the_gaps import Endpoint, Gaps
 
 
+def test_singleton_ok():
+    Gaps([1, 1])
+
+
+def test_missing_singleton_ok():
+    Gaps([0, Endpoint(1, ")"), Endpoint(1, "("), 2])
+
+
 def test_unsorted_closed():
     with pytest.raises(ValueError, match="unsorted"):
         Gaps([1, 0])
@@ -12,19 +20,31 @@ def test_unsorted_open():
         Gaps([Endpoint(1, "("), Endpoint(0, ")")])
 
 
-def test_singleton_ok():
-    Gaps([1, 1])
-
-
-def test_missing_singleton_ok():
-    Gaps([0, Endpoint(1, ")"), Endpoint(1, "("), 2])
-
-
-def test_open_closed():
+def test_not_minimal_open_closed():
     with pytest.raises(ValueError, match="not minimally expressed"):
         Gaps([0, Endpoint(1, ")"), 1, 2])
 
 
-def test_closed_open():
+def test_not_minimal_closed_open():
     with pytest.raises(ValueError, match="not minimally expressed"):
         Gaps([0, 1, Endpoint(1, "("), 2])
+
+
+def test_wrong_boundary_left_closed():
+    with pytest.raises(ValueError, match="left"):
+        Gaps([Endpoint(0, "]"), 1])
+
+
+def test_wrong_boundary_left_open():
+    with pytest.raises(ValueError, match="left"):
+        Gaps([Endpoint(0, ")"), 1])
+
+
+def test_wrong_boundary_right_closed():
+    with pytest.raises(ValueError, match="right"):
+        Gaps([0, Endpoint(1, "[")])
+
+
+def test_wrong_boundary_right_open():
+    with pytest.raises(ValueError, match="right"):
+        Gaps([0, Endpoint(1, "[")])