diff --git a/gradle-versions-plugin/src/main/kotlin/com/github/benmanes/gradle/versions/reporter/result/DependenciesGroup.kt b/gradle-versions-plugin/src/main/kotlin/com/github/benmanes/gradle/versions/reporter/result/DependenciesGroup.kt
index 67a3a5e6..01dc1d37 100644
--- a/gradle-versions-plugin/src/main/kotlin/com/github/benmanes/gradle/versions/reporter/result/DependenciesGroup.kt
+++ b/gradle-versions-plugin/src/main/kotlin/com/github/benmanes/gradle/versions/reporter/result/DependenciesGroup.kt
@@ -8,5 +8,5 @@ package com.github.benmanes.gradle.versions.reporter.result
  */
 data class DependenciesGroup<T : Dependency>(
   val count: Int,
-  val dependencies: Set<T> = sortedSetOf(),
+  val dependencies: MutableSet<T> = mutableSetOf(),
 )
diff --git a/gradle-versions-plugin/src/main/kotlin/com/github/benmanes/gradle/versions/updates/DependencyUpdatesReporter.kt b/gradle-versions-plugin/src/main/kotlin/com/github/benmanes/gradle/versions/updates/DependencyUpdatesReporter.kt
index 470774cc..a751fd68 100644
--- a/gradle-versions-plugin/src/main/kotlin/com/github/benmanes/gradle/versions/updates/DependencyUpdatesReporter.kt
+++ b/gradle-versions-plugin/src/main/kotlin/com/github/benmanes/gradle/versions/updates/DependencyUpdatesReporter.kt
@@ -176,38 +176,38 @@ class DependencyUpdatesReporter(
     )
   }
 
-  private fun buildCurrentGroup(): Set<Dependency> {
+  private fun buildCurrentGroup(): MutableSet<Dependency> {
     return sortByGroupAndName(upToDateVersions)
       .map { dep ->
         updateKey(dep.key as HashMap)
         buildDependency(dep.value, dep.key)
-      }.toSortedSet() as TreeSet<Dependency>
+      }.toSortedSet()
   }
 
-  private fun buildOutdatedGroup(): Set<DependencyOutdated> {
+  private fun buildOutdatedGroup(): MutableSet<DependencyOutdated> {
     return sortByGroupAndName(upgradeVersions)
       .map { dep ->
         updateKey(dep.key as HashMap)
         buildOutdatedDependency(dep.value, dep.key)
-      }.toSortedSet() as TreeSet<DependencyOutdated>
+      }.toSortedSet()
   }
 
-  private fun buildExceededGroup(): Set<DependencyLatest> {
+  private fun buildExceededGroup(): MutableSet<DependencyLatest> {
     return sortByGroupAndName(downgradeVersions)
       .map { dep ->
         updateKey(dep.key as HashMap)
         buildExceededDependency(dep.value, dep.key)
-      }.toSortedSet() as TreeSet<DependencyLatest>
+      }.toSortedSet()
   }
 
-  private fun buildUndeclaredGroup(): Set<Dependency> {
+  private fun buildUndeclaredGroup(): MutableSet<Dependency> {
     return undeclared
       .map { coordinate ->
         Dependency(coordinate.groupId, coordinate.artifactId)
-      }.toSortedSet() as TreeSet<Dependency>
+      }.toSortedSet()
   }
 
-  private fun buildUnresolvedGroup(): Set<DependencyUnresolved> {
+  private fun buildUnresolvedGroup(): MutableSet<DependencyUnresolved> {
     return unresolved
       .sortedWith { a, b ->
         compareKeys(keyOf(a.selector), keyOf(b.selector))
@@ -311,7 +311,7 @@ class DependencyUpdatesReporter(
       )
     }
 
-    private fun <T : Dependency> buildDependenciesGroup(dependencies: Set<T>): DependenciesGroup<T> {
+    private fun <T : Dependency> buildDependenciesGroup(dependencies: MutableSet<T>): DependenciesGroup<T> {
       return DependenciesGroup<T>(dependencies.size, dependencies)
     }
 
diff --git a/gradle-versions-plugin/src/test/groovy/com/github/benmanes/gradle/versions/OutputFormatterSpec.groovy b/gradle-versions-plugin/src/test/groovy/com/github/benmanes/gradle/versions/OutputFormatterSpec.groovy
index 3036ade1..fb592085 100644
--- a/gradle-versions-plugin/src/test/groovy/com/github/benmanes/gradle/versions/OutputFormatterSpec.groovy
+++ b/gradle-versions-plugin/src/test/groovy/com/github/benmanes/gradle/versions/OutputFormatterSpec.groovy
@@ -534,4 +534,92 @@ Failed to determine the latest version for the following dependencies (use --inf
     expected == actual
     result.task(':dependencyUpdates').outcome == SUCCESS
   }
+
+  def 'outputFormatter custom - modify unresolvable dependencies then outputs text output'() {
+    given:
+    buildFile = testProjectDir.newFile('build.gradle')
+    buildFile <<
+      """
+        import com.github.benmanes.gradle.versions.reporter.PlainTextReporter
+
+        buildscript {
+          dependencies {
+            classpath files($classpathString)
+          }
+        }
+
+        apply plugin: 'java'
+        apply plugin: 'com.github.ben-manes.versions'
+
+        repositories {
+          maven {
+            url '${mavenRepoUrl}'
+          }
+        }
+
+        dependencies {
+          implementation('backport-util-concurrent:backport-util-concurrent:3.1') { because 'I said so' }
+          implementation('backport-util-concurrent:backport-util-concurrent-java12:3.1')
+          implementation('com.google.guava:guava:99.0-SNAPSHOT') { because 'I know the future' }
+          implementation('com.google.guava:guava-tests:99.0-SNAPSHOT')
+          implementation('com.google.inject:guice:2.0') { because 'That\\'s just the way it is' }
+          implementation('com.google.inject.extensions:guice-multibindings:2.0')
+          implementation('com.github.ben-manes:unresolvable:1.0') { because 'Life is hard' }
+          implementation('com.github.ben-manes:unresolvable2:1.0')
+          implementation('com.github.ben-manes:unresolvable20:1.0')
+          implementation('com.github.ben-manes:unresolvable21:1.0')
+          implementation('com.github.ben-manes:unresolvable3:1.0')
+        }
+
+        dependencyUpdates {
+          outputFormatter = { result ->
+            result.unresolved.dependencies.removeIf {
+              it.name == 'unresolvable20' || it.name == 'unresolvable21'
+            }
+            def plainTextReporter = new PlainTextReporter(project, revision, gradleReleaseChannel)
+            plainTextReporter.write(System.out, result)
+          }
+          checkForGradleUpdate = false // future proof tests from breaking
+        }
+        """.stripIndent()
+
+    when:
+    def result = GradleRunner.create()
+      .withProjectDir(testProjectDir.root)
+      .withArguments('dependencyUpdates')
+      .withPluginClasspath()
+      .build()
+    def expected = """
+------------------------------------------------------------
+: Project Dependency Updates (report to plain text file)
+------------------------------------------------------------
+
+The following dependencies are using the latest milestone version:
+ - backport-util-concurrent:backport-util-concurrent:3.1
+     I said so
+ - backport-util-concurrent:backport-util-concurrent-java12:3.1
+
+The following dependencies exceed the version found at the milestone revision level:
+ - com.google.guava:guava [99.0-SNAPSHOT <- 16.0-rc1]
+     I know the future
+ - com.google.guava:guava-tests [99.0-SNAPSHOT <- 16.0-rc1]
+
+The following dependencies have later milestone versions:
+ - com.google.inject:guice [2.0 -> 3.1]
+     That's just the way it is
+     https://code.google.com/p/google-guice/
+ - com.google.inject.extensions:guice-multibindings [2.0 -> 3.0]
+     https://code.google.com/p/google-guice/
+
+Failed to determine the latest version for the following dependencies (use --info for details):
+ - com.github.ben-manes:unresolvable
+     Life is hard
+ - com.github.ben-manes:unresolvable2
+ - com.github.ben-manes:unresolvable3
+""".replace('\r', '').replace('\n', System.lineSeparator())
+
+    then:
+    assert result.output.toString().contains(expected)
+    result.task(':dependencyUpdates').outcome == SUCCESS
+  }
 }