diff --git a/docs/apidiffs/current_vs_latest/prometheus-metrics-instrumentation-caffeine.txt b/docs/apidiffs/current_vs_latest/prometheus-metrics-instrumentation-caffeine.txt
index 731adee45..2745dbce5 100644
--- a/docs/apidiffs/current_vs_latest/prometheus-metrics-instrumentation-caffeine.txt
+++ b/docs/apidiffs/current_vs_latest/prometheus-metrics-instrumentation-caffeine.txt
@@ -1,2 +1,6 @@
 Comparing source compatibility of prometheus-metrics-instrumentation-caffeine-1.8.1-SNAPSHOT.jar against prometheus-metrics-instrumentation-caffeine-1.8.0.jar
-No changes.
+===  UNCHANGED CLASS: PUBLIC io.prometheus.metrics.instrumentation.caffeine.CacheMetricsCollector  (not serializable)
+	===  CLASS FILE FORMAT VERSION: 52.0 <- 52.0
+	===  UNCHANGED METHOD: PUBLIC com.github.benmanes.caffeine.cache.Cache<?,?><?,?> removeCache(java.lang.String)
+		+++  NEW ANNOTATION: javax.annotation.Nullable
+
diff --git a/docs/apidiffs/current_vs_latest/prometheus-metrics-instrumentation-guava.txt b/docs/apidiffs/current_vs_latest/prometheus-metrics-instrumentation-guava.txt
index 7391e926c..bb87fbec9 100644
--- a/docs/apidiffs/current_vs_latest/prometheus-metrics-instrumentation-guava.txt
+++ b/docs/apidiffs/current_vs_latest/prometheus-metrics-instrumentation-guava.txt
@@ -1,2 +1,6 @@
 Comparing source compatibility of prometheus-metrics-instrumentation-guava-1.8.1-SNAPSHOT.jar against prometheus-metrics-instrumentation-guava-1.8.0.jar
-No changes.
+===  UNCHANGED CLASS: PUBLIC io.prometheus.metrics.instrumentation.guava.CacheMetricsCollector  (not serializable)
+	===  CLASS FILE FORMAT VERSION: 52.0 <- 52.0
+	===  UNCHANGED METHOD: PUBLIC com.google.common.cache.Cache<?,?><?,?> removeCache(java.lang.String)
+		+++  NEW ANNOTATION: javax.annotation.Nullable
+
diff --git a/pom.xml b/pom.xml
index 5752f4ce9..f9126506f 100644
--- a/pom.xml
+++ b/pom.xml
@@ -505,7 +505,7 @@
                 <path>
                   <groupId>com.uber.nullaway</groupId>
                   <artifactId>nullaway</artifactId>
-                  <version>0.13.7</version>
+                  <version>0.13.8</version>
                 </path>
               </annotationProcessorPaths>
             </configuration>
diff --git a/prometheus-metrics-instrumentation-caffeine/src/main/java/io/prometheus/metrics/instrumentation/caffeine/CacheMetricsCollector.java b/prometheus-metrics-instrumentation-caffeine/src/main/java/io/prometheus/metrics/instrumentation/caffeine/CacheMetricsCollector.java
index ba7552910..687cd4349 100644
--- a/prometheus-metrics-instrumentation-caffeine/src/main/java/io/prometheus/metrics/instrumentation/caffeine/CacheMetricsCollector.java
+++ b/prometheus-metrics-instrumentation-caffeine/src/main/java/io/prometheus/metrics/instrumentation/caffeine/CacheMetricsCollector.java
@@ -20,6 +20,7 @@
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 import java.util.stream.Collectors;
+import javax.annotation.Nullable;
 
 /**
  * Collect metrics from Caffeine's com.github.benmanes.caffeine.cache.Cache.
@@ -149,6 +150,7 @@ public void addCache(String cacheName, AsyncCache<?, ?> cache) {
    *
    * @param cacheName cache to be removed
    */
+  @Nullable
   public Cache<?, ?> removeCache(String cacheName) {
     return children.remove(cacheName);
   }
diff --git a/prometheus-metrics-instrumentation-guava/src/main/java/io/prometheus/metrics/instrumentation/guava/CacheMetricsCollector.java b/prometheus-metrics-instrumentation-guava/src/main/java/io/prometheus/metrics/instrumentation/guava/CacheMetricsCollector.java
index b807a28f0..0b6ff3d78 100644
--- a/prometheus-metrics-instrumentation-guava/src/main/java/io/prometheus/metrics/instrumentation/guava/CacheMetricsCollector.java
+++ b/prometheus-metrics-instrumentation-guava/src/main/java/io/prometheus/metrics/instrumentation/guava/CacheMetricsCollector.java
@@ -16,6 +16,7 @@
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
+import javax.annotation.Nullable;
 
 /**
  * Collect metrics from Guava's com.google.common.cache.Cache.
@@ -100,6 +101,7 @@ public void addCache(String cacheName, Cache<?, ?> cache) {
    *
    * @param cacheName cache to be removed
    */
+  @Nullable
   public Cache<?, ?> removeCache(String cacheName) {
     return children.remove(cacheName);
   }