diff --git a/kmongo/src/main/kotlin/org/litote/kmongo/extensions.kt b/kmongo/src/main/kotlin/org/litote/kmongo/extensions.kt
index 1b3188a6..0b274b54 100644
--- a/kmongo/src/main/kotlin/org/litote/kmongo/extensions.kt
+++ b/kmongo/src/main/kotlin/org/litote/kmongo/extensions.kt
@@ -470,7 +470,7 @@ fun <T> MongoCollection<T>.findOneAndDelete(filter: String, options: FindOneAndD
  * document as it was before the update or as it is after the update.  If no documents matched the query filter, then null will be
  * returned
  */
-fun <T> MongoCollection<T>.findOneAndReplace(filter: String, replacement: T, options: FindOneAndReplaceOptions = FindOneAndReplaceOptions()): T
+fun <T> MongoCollection<T>.findOneAndReplace(filter: String, replacement: T, options: FindOneAndReplaceOptions = FindOneAndReplaceOptions()): T?
         = findOneAndReplace(toBson(filter), replacement, options)
 
 /**
@@ -484,7 +484,7 @@ fun <T> MongoCollection<T>.findOneAndReplace(filter: String, replacement: T, opt
  * document as it was before the update or as it is after the update.  If no documents matched the query filter, then null will be
  * returned
  */
-fun <T> MongoCollection<T>.findOneAndUpdate(filter: String, update: String, options: FindOneAndUpdateOptions = FindOneAndUpdateOptions()): T
+fun <T> MongoCollection<T>.findOneAndUpdate(filter: String, update: String, options: FindOneAndUpdateOptions = FindOneAndUpdateOptions()): T?
         = findOneAndUpdate(toBson(filter), toBson(update), options)
 
 /**
diff --git a/kmongo/src/test/kotlin/org/litote/kmongo/issues/Issue34FindOneAndUpdateOrReplaceCouldReturnNull.kt b/kmongo/src/test/kotlin/org/litote/kmongo/issues/Issue34FindOneAndUpdateOrReplaceCouldReturnNull.kt
new file mode 100644
index 00000000..c26fda1d
--- /dev/null
+++ b/kmongo/src/test/kotlin/org/litote/kmongo/issues/Issue34FindOneAndUpdateOrReplaceCouldReturnNull.kt
@@ -0,0 +1,26 @@
+package org.litote.kmongo.issues
+
+import org.junit.Test
+import org.litote.kmongo.KMongoBaseTest
+import org.litote.kmongo.MongoOperator.set
+import org.litote.kmongo.findOneAndReplace
+import org.litote.kmongo.findOneAndUpdate
+import org.litote.kmongo.model.Friend
+import kotlin.test.assertNull
+
+/**
+ *
+ */
+class Issue34FindOneAndUpdateOrReplaceCouldReturnNull : KMongoBaseTest<Friend>() {
+
+    @Test
+    fun findOneAndReplaceCouldReturnsNull() {
+        assertNull(col.findOneAndReplace("{}", Friend("test")))
+    }
+
+    @Test
+    fun findOneAndUpdateCouldReturnsNull() {
+        assertNull(col.findOneAndUpdate("{}", "{$set:{name:'John'}}"))
+    }
+
+}
\ No newline at end of file