diff --git a/gradle/libs.versions.toml b/gradle/libs.versions.toml
index d34b443904..b6c1cac15f 100644
--- a/gradle/libs.versions.toml
+++ b/gradle/libs.versions.toml
@@ -7,7 +7,7 @@ coroutines = "1.11.0"
 dokka = "2.2.0"
 grpc = "1.81.0"
 gson = "2.14.0"
-guava = "32.1.3-jre"
+guava = "33.6.0-jre"
 javapoet = "0.16.0"
 jimfs = "1.3.1"
 jmh = "1.37"
diff --git a/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/Multimap.kt b/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/Multimap.kt
index eab31d1e99..a9b5ef0c6d 100644
--- a/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/Multimap.kt
+++ b/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/Multimap.kt
@@ -21,7 +21,7 @@ expect interface Multimap<K, V> {
   fun containsKey(key: Any?): Boolean
   fun containsValue(value: Any?): Boolean
 
-  operator fun get(key: K?): Collection<V>
+  operator fun get(key: K): Collection<V>
   fun values(): Collection<V>
   fun asMap(): Map<K, Collection<V>>
 }
diff --git a/wire-schema/src/jsMain/kotlin/com/squareup/wire/schema/Multimap.kt b/wire-schema/src/jsMain/kotlin/com/squareup/wire/schema/Multimap.kt
index 2b82fbe91d..a78bfd1384 100644
--- a/wire-schema/src/jsMain/kotlin/com/squareup/wire/schema/Multimap.kt
+++ b/wire-schema/src/jsMain/kotlin/com/squareup/wire/schema/Multimap.kt
@@ -20,7 +20,7 @@ actual interface Multimap<K, V> {
   actual fun isEmpty(): Boolean
   actual fun containsKey(key: Any?): Boolean
   actual fun containsValue(value: Any?): Boolean
-  actual operator fun get(key: K?): Collection<V>
+  actual operator fun get(key: K): Collection<V>
   actual fun values(): Collection<V>
   actual fun asMap(): Map<K, Collection<V>>
 }
@@ -30,7 +30,7 @@ private class SimpleMultimap<K, V>(private val map: Map<K, Collection<V>>) : Mul
   override fun isEmpty() = map.isEmpty()
   override fun containsKey(key: Any?) = map.containsKey(key)
   override fun containsValue(value: Any?) = map.values.any { it.contains(value) }
-  override fun get(key: K?) = map[key] ?: emptyList()
+  override fun get(key: K) = map[key] ?: emptyList()
   override fun values() = map.values.flatten()
   override fun asMap() = map
 }
diff --git a/wire-schema/src/nativeMain/kotlin/com/squareup/wire/schema/Multimap.kt b/wire-schema/src/nativeMain/kotlin/com/squareup/wire/schema/Multimap.kt
index b23c3d2c2a..c448d03425 100644
--- a/wire-schema/src/nativeMain/kotlin/com/squareup/wire/schema/Multimap.kt
+++ b/wire-schema/src/nativeMain/kotlin/com/squareup/wire/schema/Multimap.kt
@@ -20,7 +20,7 @@ actual interface Multimap<K, V> {
   actual fun isEmpty(): Boolean
   actual fun containsKey(key: Any?): Boolean
   actual fun containsValue(value: Any?): Boolean
-  actual operator fun get(key: K?): Collection<V>
+  actual operator fun get(key: K): Collection<V>
   actual fun values(): Collection<V>
   actual fun asMap(): Map<K, Collection<V>>
 }
@@ -30,7 +30,7 @@ private class SimpleMultimap<K, V>(private val map: Map<K, Collection<V>>) : Mul
   override fun isEmpty() = map.isEmpty()
   override fun containsKey(key: Any?) = map.containsKey(key)
   override fun containsValue(value: Any?) = map.values.any { it.contains(value) }
-  override fun get(key: K?) = map[key] ?: emptyList()
+  override fun get(key: K) = map[key] ?: emptyList()
   override fun values() = map.values.flatten()
   override fun asMap() = map
 }