diff --git a/graphene/validation/depth_limit.py b/graphene/validation/depth_limit.py
index e0f286634..ac44818a7 100644
--- a/graphene/validation/depth_limit.py
+++ b/graphene/validation/depth_limit.py
@@ -144,8 +144,11 @@ def determine_depth(
             )
         )
     elif isinstance(node, FragmentSpreadNode):
+        fragment = fragments.get(node.name.value)
+        if fragment is None:
+            return 0
         return determine_depth(
-            node=fragments[node.name.value],
+            node=fragment,
             fragments=fragments,
             depth_so_far=depth_so_far,
             max_depth=max_depth,
diff --git a/graphene/validation/tests/test_depth_limit_validator.py b/graphene/validation/tests/test_depth_limit_validator.py
index 29c1508c4..20e0e3d4b 100644
--- a/graphene/validation/tests/test_depth_limit_validator.py
+++ b/graphene/validation/tests/test_depth_limit_validator.py
@@ -244,6 +244,19 @@ def test_should_ignore_field():
     assert result == expected
 
 
+def test_undefined_fragment_does_not_crash():
+    query = """
+    query read1 {
+      user {
+        name
+        ...undefinedFragment
+      }
+    }
+    """
+    errors, result = run_query(query, 10)
+    assert result == {"read1": 1}
+
+
 def test_should_raise_invalid_ignore():
     query = """
     query read1 {