diff --git a/crates/dav-proto/src/schema/request.rs b/crates/dav-proto/src/schema/request.rs
index 397b07d0..878c03e6 100644
--- a/crates/dav-proto/src/schema/request.rs
+++ b/crates/dav-proto/src/schema/request.rs
@@ -5,18 +5,18 @@
  */
 
 use super::{
+    Collation, MatchType,
     property::{DavProperty, DavValue, LockScope, LockType},
     response::Ace,
-    Collation, MatchType,
 };
-use crate::Depth;
+use crate::{Condition, Depth};
 use calcard::{
     icalendar::{ICalendarComponentType, ICalendarParameterName, ICalendarProperty},
     vcard::{VCardParameterName, VCardProperty},
 };
 use types::{
-    dead_property::{ArchivedDeadProperty, ArchivedDeadPropertyTag, DeadElementTag, DeadProperty},
     TimeRange,
+    dead_property::{ArchivedDeadProperty, ArchivedDeadPropertyTag, DeadElementTag, DeadProperty},
 };
 
 #[derive(Debug, Clone, PartialEq, Eq, Default)]
@@ -284,3 +284,12 @@ impl DavDeadProperty for ArchivedDeadProperty {
         }
     }
 }
+
+impl Condition<'_> {
+    pub fn is_none_match(&self) -> bool {
+        match self {
+            Condition::ETag { is_not, .. } | Condition::Exists { is_not } => *is_not,
+            Condition::StateToken { .. } => false,
+        }
+    }
+}
diff --git a/crates/dav/src/common/lock.rs b/crates/dav/src/common/lock.rs
index 75b1daef..0302cb78 100644
--- a/crates/dav/src/common/lock.rs
+++ b/crates/dav/src/common/lock.rs
@@ -605,7 +605,18 @@ impl LockRequestHandler for Server {
             return lock_response;
         }
 
-        Err(DavError::Code(StatusCode::PRECONDITION_FAILED))
+        Err(DavError::Code(
+            if matches!(method, DavMethod::GET | DavMethod::HEAD)
+                && headers
+                    .if_
+                    .iter()
+                    .any(|if_| if_.list.iter().any(|cond| cond.is_none_match()))
+            {
+                StatusCode::NOT_MODIFIED
+            } else {
+                StatusCode::PRECONDITION_FAILED
+            },
+        ))
     }
 }